Factors modifying drug actions

Reading File

Finding Sources

Reading File

Finding Sources

Reading File

Finding Sources

I now have enough material from the library. Let me compile a thorough answer.

Factors Modifying Drug Actions

Drug action is not uniform across all individuals or all circumstances. Many patient-specific, physiological, genetic, and environmental factors can alter a drug's pharmacokinetics (what the body does to the drug) or pharmacodynamics (what the drug does to the body), resulting in variable responses - from therapeutic success to toxicity or complete failure.

1. Pharmacogenetic and Genetic Factors

Individual drug response depends on the complex interplay of drug factors (dose, route, formulation), environmental factors (diet, infections, other drugs, exercise, toxins), clinical factors (age, organ function), and genetic factors.

Key genes involved in driving variable drug actions are called pharmacogenes. They influence drug response via:

Pharmacokinetics - variable drug concentrations
Pharmacodynamics - variable drug effects

Pharmacogenes encode:

Drug-metabolizing enzymes (e.g., CYP2D6, CYP2C19, CYP2C9)
Drug transport molecules (e.g., P-glycoprotein, SLCO1B1)
Drug targets (e.g., beta-adrenoceptor variants)
Genes modulating the molecular context of disease

Metabolizer Phenotypes

Drug metabolism traits can follow a "monogenic" pattern, producing distinct groups:

Phenotype	Genotype	Clinical Effect
Poor metabolizer	Two non-functional alleles	Drug accumulates → toxicity risk
Intermediate metabolizer	One functional + one non-functional allele	Reduced clearance
Normal (extensive) metabolizer	Two functional alleles	Standard response
Ultra-rapid metabolizer	Gene duplication	Rapid clearance → therapeutic failure

Classic Examples

G6PD deficiency - Hemolysis with antimalarials (primaquine), sulfonamides (pharmacodynamic)
Pseudocholinesterase deficiency - Prolonged paralysis after succinylcholine
Malignant hyperthermia - Ryanodine receptor (RYR1) mutation with volatile anaesthetics
NAT (N-acetyltransferase) polymorphism - Slow vs. fast acetylators affect INH, hydralazine, procainamide
CYP2D6 variants - Affect codeine, metoprolol, tricyclic antidepressants, tamoxifen

Source: Goodman & Gilman's The Pharmacological Basis of Therapeutics

2. Age

Age profoundly affects both pharmacokinetics and pharmacodynamics.

Neonates and Infants

Absorption: Gastric pH is higher (less acid), affecting absorption of weakly acidic/basic drugs; slower gastric emptying
Distribution: Higher body water, lower plasma protein (albumin), higher free drug fractions; immature blood-brain barrier
Metabolism: Hepatic CYP enzymes immature at birth (especially CYP3A4, glucuronidation); chloramphenicol "Grey Baby Syndrome" - underdeveloped conjugation enzymes lead to toxic accumulation
Excretion: Glomerular filtration rate (GFR) is ~30% of adult value at birth; full maturation by 6-12 months

Elderly

Absorption: Reduced gut motility; reduced first-pass metabolism
Distribution: Reduced lean body mass, increased fat (increased Vd for lipid-soluble drugs); reduced albumin (higher free drug)
Metabolism: Hepatic blood flow and enzyme activity decline by ~30-40%
Excretion: GFR declines with age (~1 mL/min per year after 40); serum creatinine may not reflect this due to reduced muscle mass
Pharmacodynamics: CNS more sensitive to sedatives, opioids, anticoagulants; polypharmacy increases interaction risk

3. Body Weight and Body Composition

Doses for obese patients: fat-soluble drugs (e.g., diazepam) have a larger volume of distribution
Water-soluble drugs (e.g., aminoglycosides) are dosed on lean body weight, not total body weight
Doses in pediatrics are typically calculated in mg/kg or by body surface area (mg/m²) for chemotherapy

4. Sex (Biological)

Women generally have lower body weight, less lean mass, lower CYP3A4 activity (though CYP3A4 is actually slightly higher in women)
CYP2D6 and CYP2C19 show some sex-based differences
Women are more sensitive to some CNS drugs (anesthetics, analgesics)
QT prolongation risk from antiarrhythmics (e.g., sotalol, quinidine) is greater in women due to longer baseline QT
Hormonal changes in pregnancy significantly alter pharmacokinetics (see below)

5. Pregnancy

Absorption: Delayed gastric emptying, nausea/vomiting alter oral bioavailability
Distribution: Plasma volume expands by ~50%, altering Vd; albumin falls (more free drug); additional compartment (fetal-placental)
Metabolism: CYP3A4, CYP2C9 activity increases; CYP1A2 and CYP2C19 decrease
Excretion: GFR increases by ~50%; renal drug clearance increases (e.g., ampicillin, digoxin need dose adjustment)
Fetal risk: Teratogens act on the fetus via placental transfer (e.g., thalidomide, phenytoin, warfarin)

6. Disease States

Renal Disease

Reduced clearance of renally excreted drugs and active metabolites
Dose reduction or interval extension required (creatinine clearance guides adjustment)
Examples: aminoglycosides, digoxin, NSAIDs (risk of acute kidney injury), lithium, metformin (contraindicated in severe CKD)

Hepatic Disease

Reduced first-pass metabolism → increased bioavailability of oral drugs (e.g., propranolol, morphine)
Reduced synthesis of plasma proteins (lower albumin, higher free drug fraction)
Portosystemic shunts bypass hepatic metabolism entirely
Reduced bile production affects absorption of fat-soluble drugs and enterohepatic circulation
Examples requiring caution: benzodiazepines, opioids, paracetamol, warfarin

Gastrointestinal Disease

Malabsorption (Crohn's disease, coeliac disease) reduces drug absorption
Altered gut motility (diarrhoea or constipation) affects absorption rate

Cardiovascular Disease

Reduced cardiac output → reduced hepatic and renal blood flow → reduced drug clearance
Edematous states alter Vd of water-soluble drugs

Thyroid Disease

Hypothyroidism slows metabolism; hyperthyroidism increases it
Hypothyroidism increases sensitivity to digoxin; hyperthyroidism decreases it

7. Drug Interactions

Drugs can modify each other's actions at multiple levels:

Pharmacokinetic Interactions

Absorption: Antacids reduce absorption of tetracyclines, fluoroquinolones (chelation); metoclopramide increases gastric emptying (faster absorption)
Protein binding displacement: Warfarin displaced by NSAIDs - transient increase in free warfarin
Enzyme induction: Rifampicin, carbamazepine, phenytoin induce CYP enzymes → reduced levels of co-administered drugs (OCP failure, reduced anticoagulation)
Enzyme inhibition: Erythromycin, ketoconazole, grapefruit juice inhibit CYP3A4 → increased drug levels and toxicity (e.g., statins → myopathy; ciclosporin toxicity)
Transporter interactions: P-gp inhibitors (e.g., amiodarone) raise digoxin levels

Pharmacodynamic Interactions

Synergism/Additive: Alcohol + benzodiazepines (CNS depression); aspirin + heparin (additive bleeding)
Antagonism: Beta-blockers antagonize salbutamol; naloxone reverses opioids
Receptor level: Warfarin and vitamin K compete at clotting factor synthesis

8. Route and Formulation of Administration

Oral: Subject to first-pass metabolism; slower onset; affected by food, gastric pH, motility
Intravenous: 100% bioavailability; immediate onset; no first-pass effect
Sublingual: Bypasses first-pass (e.g., glyceryl trinitrate, buprenorphine)
Transdermal: Slow, sustained delivery; bypasses GI and first-pass (e.g., fentanyl patch, nicotine patch)
Inhaled: Rapid onset for pulmonary/systemic effects; local action in airways (e.g., salbutamol)
Modified release formulations (SR/XR/CR): Alter release kinetics, reduce dosing frequency, smooth concentration-time profiles

Food interactions:

High-fat meals increase absorption of lipophilic drugs (e.g., griseofulvin, itraconazole)
Grapefruit juice irreversibly inhibits intestinal CYP3A4
Tyramine-rich foods + MAOIs → hypertensive crisis

9. Tolerance, Tachyphylaxis, and Dependence

Term	Definition	Example
Tolerance	Reduced response to a drug on repeated administration; requires increasing dose for same effect	Opioids, nitrates, benzodiazepines
Tachyphylaxis	Rapid tolerance developing after just a few doses (acute tolerance)	Ephedrine, LSD, amphetamines
Cross-tolerance	Tolerance to one drug confers tolerance to pharmacologically related drugs	Opioids, benzodiazepines
Dependence	Physical/psychological need for drug; withdrawal syndrome on cessation	Opioids, alcohol, benzodiazepines

Mechanisms of tolerance:

Receptor downregulation (decreased receptor density)
Receptor desensitization (uncoupling of receptor from effector)
Increased metabolic degradation (pharmacokinetic tolerance)
Compensatory physiological mechanisms (counterregulation)

10. Psychological Factors - Placebo Effect

A placebo (inert substance) can produce measurable physiological effects through expectation and conditioning
The placebo effect can account for 30-40% of the analgesic response in clinical trials
Nocebo effect: Negative expectations worsen outcomes or produce side effects
These factors are accounted for in double-blind, randomized controlled trials

11. Cumulation and Dose-Related Factors

Drugs with long half-lives or narrow therapeutic indices accumulate with repeated dosing if elimination is impaired
Steady-state concentration is reached in ~5 half-lives; influenced by dose, frequency, and clearance
Therapeutic drug monitoring (TDM) is used for drugs with narrow therapeutic windows (e.g., digoxin, phenytoin, lithium, aminoglycosides, vancomycin)

12. Environmental and Lifestyle Factors

Factor	Effect
Smoking	Induces CYP1A2 → reduces levels of theophylline, clozapine, olanzapine
Alcohol (chronic use)	Induces CYP2E1 → increased paracetamol hepatotoxicity
Alcohol (acute use)	Inhibits CYP enzymes → increases drug levels
Diet	High-protein diet induces drug metabolism; grapefruit CYP3A4 inhibition
Exercise	Alters blood flow, absorption, and distribution
Altitude	Alters drug absorption (hypoxia-induced GI changes)

Summary Table: Key Factors at a Glance

Factor	Primary Pharmacokinetic Impact	Pharmacodynamic Impact
Age (pediatric/elderly)	Altered metabolism & excretion	Altered receptor sensitivity
Renal disease	Reduced excretion	-
Hepatic disease	Reduced metabolism, protein binding	-
Genetics (CYP polymorphism)	Altered metabolism	Altered receptor variants
Pregnancy	Increased Vd, altered metabolism	Teratogenicity
Drug interactions	ADME alterations	Synergism/antagonism
Route of administration	Bioavailability, onset	-
Tolerance	-	Receptor downregulation
Body weight/composition	Vd alteration	-
Smoking/diet	Enzyme induction/inhibition	-

Understanding these modifying factors is fundamental to individualized (precision) pharmacotherapy - selecting the right drug, at the right dose, via the right route, for the right patient.

Sources: Goodman & Gilman's The Pharmacological Basis of Therapeutics; Katzung's Basic and Clinical Pharmacology, 16th Edition; Goldman-Cecil Medicine

Routes of drug administration

Reading File

Finding Sources

Reading File

Loading Image

I have comprehensive content from both Lippincott and Goodman & Gilman, plus an excellent diagram. Here is the full answer:

Routes of Drug Administration

The route of administration is determined by the drug's physicochemical properties (water/lipid solubility, ionization, molecular size, stability) and the therapeutic objective (speed of onset, duration, local vs systemic effect, patient condition).

Figure: Commonly used routes of drug administration - Lippincott Illustrated Reviews: Pharmacology

Routes are broadly classified into three categories: Enteral, Parenteral, and Other (Topical/Special).

I. ENTERAL ROUTES

Enteral administration involves the GI tract. It is the most common, convenient, and economical method.

1. Oral (Per Oral / PO)

Mechanism of absorption: Primarily passive diffusion through the GI mucosa. Most absorption occurs in the small intestine due to its large surface area and rich blood supply. Nonionized, lipophilic drugs are better absorbed.

Bioavailability (F): Variable - 0.05 < F < 1, highly dependent on drug properties and GI factors.

Advantages:

Safest, most convenient, most economical
Self-administration possible
Large variety of formulations available
Overdose can be managed (e.g., activated charcoal, gastric lavage)

Disadvantages:

Irregular and incomplete absorption for some drugs
Inactivation by gastric acid (e.g., penicillin G, insulin)
Destruction by digestive enzymes
First-pass metabolism by intestinal mucosa, gut microbiome, and liver - significantly reduces bioavailability
Requires patient cooperation/compliance
Vomiting and food can alter absorption
Unsuitable for unconscious or vomiting patients

Factors affecting GI absorption:

Surface area and blood flow at absorption site
Drug solubility (rate-limiting step for solid forms is dissolution)
Gastric pH (weak acids absorbed better in stomach pH 1-2; weak bases absorbed better in intestine pH 5-7)
Gastric emptying rate: faster emptying generally increases absorption rate
Presence of food (can delay, reduce, or increase absorption)
Gut microbiome (>1000 species can metabolize drugs)

Special oral formulations:

Formulation	Mechanism	Purpose	Example
Enteric-coated	Chemical coating dissolves in alkaline intestine, not acid stomach	Protect acid-labile drugs; avoid gastric irritation	Omeprazole, aspirin EC
Extended-release (ER/XR/SR/CR)	Special coatings/matrix control drug release rate	Slower absorption, prolonged action, fewer doses/day, smoother plasma levels, better compliance	Morphine SR, metoprolol XL, nifedipine CR

Controlled-release preparations are most suitable for drugs with short half-lives (t½ < 4 h). - Goodman & Gilman's

2. Sublingual

Drug is placed under the tongue. Absorbed directly into systemic venous circulation through the highly vascular sublingual mucosa.

Bioavailability: High for suitable drugs (e.g., nitroglycerin ~100%)

Advantages:

Rapid onset of action (onset within 1-2 minutes for nitroglycerin)
Completely bypasses first-pass hepatic metabolism
Avoids destruction by gastric acid and digestive enzymes
Saliva pH is ~7 (neutral) - drug stability maintained

Disadvantages:

Limited to small doses
Only suitable for drugs with high lipid solubility and low molecular weight
Some drug is lost by swallowing saliva

Examples: Nitroglycerin (angina), buprenorphine, fentanyl (pain), ergotamine

3. Buccal

Drug is placed between the cheek and gum. Similar advantages to sublingual - bypasses first-pass metabolism and gastric acid.

Examples: Buccal midazolam (for seizures), testosterone buccal tablets

4. Rectal

Drug is administered via suppository or enema into the rectum.

Key pharmacokinetic point: Approximately 50% of the rectal venous drainage bypasses the portal circulation (via inferior and middle rectal veins → internal iliac → inferior vena cava), reducing but not eliminating first-pass metabolism.

Advantages:

Useful when patient is vomiting, unconscious, or unable to swallow
Avoids GI enzyme destruction
Partial bypass of first-pass effect
Useful for local anorectal conditions

Disadvantages:

Absorption is erratic and incomplete
Many drugs irritate the rectal mucosa
Psychologically unacceptable to some patients

Examples: Diazepam (status epilepticus in children), paracetamol, mesalamine (ulcerative colitis), ondansetron

II. PARENTERAL ROUTES

Parenteral means "outside the alimentary canal." Drugs are introduced directly into tissues or the bloodstream, bypassing the GI tract entirely.

General indications for parenteral use:

Drugs poorly absorbed or unstable in the GI tract (heparin, insulin, aminoglycosides)
Unconscious or uncooperative patients
Need for rapid onset (emergencies)
Need for precise dose control

General disadvantages:

Irreversible once administered
Requires sterile technique (risk of infection)
Pain, fear, local tissue damage
Trained personnel usually required (except SC/IM self-injection)
Cannot be recalled or neutralized like oral overdose

1. Intravenous (IV)

Drug is injected directly into a vein.

Bioavailability: F = 1.0 by definition (100% - absorption is circumvented).

Modes:

IV bolus: Entire dose delivered rapidly; highest immediate plasma concentration; used in emergencies
IV infusion: Drug infused over time; lower peak concentrations; more controlled; used for drugs requiring steady plasma levels (e.g., heparin, vancomycin, chemotherapy)

Advantages:

Fastest onset of action (seconds)
Maximum control over plasma levels; allows precise dose titration
Suitable for large volumes
Suitable for irritating drugs (diluted in large volume)
Suitable for high-molecular-weight proteins/peptides (e.g., monoclonal antibodies)
Essential in emergencies

Disadvantages:

No recall possible - adverse reactions immediate and severe
Risk of thrombophlebitis, embolism, infection (septicaemia)
Must inject slowly as a rule (to avoid cardiovascular collapse)
Oily solutions or poorly water-soluble drugs cannot be given IV
Strict aseptic technique required

Examples: Morphine (acute pain/MI), furosemide (pulmonary oedema), adrenaline (anaphylaxis), most chemotherapy agents

2. Intramuscular (IM)

Drug is injected into skeletal muscle (deltoid, gluteus maximus, vastus lateralis).

Bioavailability: 0.75 < F < 1

Absorption: Via simple diffusion into capillaries; rate depends on:

Drug solubility: aqueous solutions absorbed rapidly; oily/depot preparations absorbed slowly and sustainedly
Blood flow to the muscle (reduced in shock)

Depot preparations: Drug suspended in non-aqueous vehicle (oil, polyethylene glycol). Vehicle diffuses out, drug precipitates at site, then dissolves slowly - provides sustained drug release over days to weeks.

Advantages:

Moderate volumes can be given (up to 5 mL)
Suitable for oily vehicles and some irritating substances
Depot formulations provide prolonged effect (useful for antipsychotics, contraceptives)
Suitable for self-injection (e.g., insulin, some vaccines)

Disadvantages:

Possible pain or necrosis with irritating drugs
Contraindicated during anticoagulant therapy (risk of haematoma)
Absorption unreliable in shock (reduced blood flow)
May falsely elevate creatine kinase (CK) on blood tests
Accidental intravenous injection possible

Examples: Depot antipsychotics (fluphenazine decanoate, haloperidol decanoate), depo-medroxyprogesterone (contraception), IM vaccines, penicillin G benzathine

3. Subcutaneous (SC / SQ)

Drug is injected into the loose connective tissue beneath the skin.

Bioavailability: 0.75 < F < 1

Absorption: Via simple diffusion; slower than IM due to less vascularity; can be further slowed by adding vasoconstrictors (e.g., adrenaline with local anaesthetics).

Advantages:

Slow, constant, sustained absorption
Less risk of haemolysis or thrombosis compared to IV
Suitable for self-injection
Can be used for implants (e.g., hormonal implants, insulin pumps)

Disadvantages:

Small volumes only (~2 mL maximum)
Not suitable for irritating or hypertonic solutions (severe pain and tissue necrosis)
Absorption unreliable in poor peripheral perfusion

Examples: Insulin, heparin (LMWH), adrenaline (anaphylaxis kit), some vaccines, hormonal implants (etonogestrel)

4. Intradermal (ID)

Drug is injected into the dermis (more vascular layer beneath the epidermis), producing a characteristic bleb.

Volume: Very small (0.1 mL)

Uses:

Allergy skin testing (tuberculin/Mantoux test, allergen testing)
Diagnostic skin tests
Desensitisation (immunotherapy)
BCG vaccine

Examples: Tuberculin (PPD), BCG, allergen testing solutions

5. Intra-arterial

Drug is injected directly into an artery to deliver high concentrations to a specific organ before systemic dilution.

Uses:

Intra-arterial chemotherapy (e.g., hepatic artery infusion for liver metastases)
Contrast dye injection for angiography
Thrombolytic therapy (e.g., intra-arterial tPA for stroke)

Risk: Serious - can cause arterial spasm, thrombosis, ischaemia distally

6. Intrathecal / Intraventricular

Drug is injected into the subarachnoid space (intrathecal) or directly into a cerebral ventricle (intraventricular).

Rationale: The blood-brain barrier (BBB) prevents many drugs from reaching the CNS. Intrathecal delivery bypasses the BBB entirely.

Uses:

Spinal anaesthesia (bupivacaine, lidocaine)
Intrathecal chemotherapy (methotrexate for CNS lymphoma/leukaemia)
Antibiotics for meningitis when systemic levels are insufficient
Intrathecal baclofen (severe spasticity)
Opioids for intractable pain

7. Intraosseous (IO)

Drug or fluid is injected directly into the bone marrow cavity (usually tibial plateau or sternum).

Use: Emergency vascular access when IV access is impossible (e.g., paediatric resuscitation, cardiac arrest). Bone marrow communicates with central circulation rapidly.

8. Epidural / Intradural

Drug is injected into the epidural space (between the ligamentum flavum and dura mater).

Uses:

Epidural anaesthesia/analgesia in labour and surgery
Epidural corticosteroid injections for disc prolapse/radicular pain
Epidural opioids for postoperative/cancer pain

III. TOPICAL AND OTHER ROUTES

1. Topical

Drug is applied to skin or mucous membranes for local action at the site of application, with minimal systemic absorption desired.

Examples: Corticosteroid creams (eczema), antifungal creams, eye drops, ear drops, nasal sprays (for local rhinitis), mouth washes

2. Transdermal

Drug is applied to skin (usually via a patch) for systemic absorption.

Mechanism: Drug diffuses through skin layers into dermal capillaries. Rate of absorption depends on:

Lipid solubility of the drug (must penetrate the stratum corneum)
Skin thickness and integrity at the application site
Surface area of contact
Drug concentration in the vehicle

Advantages:

Avoids first-pass metabolism
Prolonged, steady plasma levels (zero-order kinetics)
Non-invasive, convenient, improved compliance
Easily removable in case of adverse reaction

Disadvantages:

Only suitable for potent, lipid-soluble drugs
Slow onset (hours to reach therapeutic levels)
Local skin irritation or allergic contact dermatitis
Cannot deliver large doses

Examples:

Drug	Indication	Patch Duration
Glyceryl trinitrate (GTN)	Angina prophylaxis	24 hours
Fentanyl	Chronic cancer/non-cancer pain	72 hours
Nicotine	Smoking cessation	16-24 hours
Estradiol	HRT, contraception	Variable
Buprenorphine	Pain	7 days
Clonidine	Hypertension	7 days
Scopolamine	Motion sickness	3 days

3. Inhalation

Drug is inhaled as a gas, vapour, or aerosol to act locally in the lungs or for rapid systemic absorption.

Why rapid? The pulmonary epithelium has an enormous surface area (~70 m²) and an extremely rich blood supply; drug crosses a very thin membrane to reach the circulation - onset nearly as fast as IV bolus.

Forms:

Pressurised metered-dose inhalers (pMDI)
Dry powder inhalers (DPI)
Nebulisers
Volatile anaesthetic gases

Advantages:

Rapid onset
Local delivery to the lung - high local concentrations with low systemic side effects (e.g., inhaled corticosteroids)
Effective for gases (volatile anaesthetics, oxygen)

Disadvantages:

Requires patient coordination and technique (pMDI)
Dose difficult to calculate precisely
Aerosol deposition unpredictable
Local adverse effects (candidiasis with inhaled steroids)

Examples: Salbutamol (asthma), beclometasone (inhaled steroid), tiotropium (COPD), volatile anaesthetics (isoflurane, sevoflurane)

4. Nasal

Drug is administered directly into the nasal cavity.

Uses:

Local: allergic rhinitis (fluticasone nasal spray), nasal decongestants (xylometazoline)
Systemic: desmopressin (diabetes insipidus), calcitonin, sumatriptan, midazolam (for seizures - intranasal), naloxone (overdose reversal)

Advantage: Avoids first-pass metabolism; rapid onset for systemic drugs (highly vascular nasal mucosa).

5. Ocular

Drug is instilled as eye drops or ointment into the conjunctival sac.

Primarily for local effect (glaucoma drops, antibiotics for conjunctivitis) but systemic absorption can occur via the nasolacrimal duct (clinical significance: beta-blocker eye drops causing bradycardia/bronchospasm).

6. Vaginal

Drugs administered as pessaries, creams, or rings for local or systemic effect.

Examples: Clotrimazole (local antifungal), misoprostol (cervical ripening), progesterone gel (IVF support), combined hormonal vaginal ring (NuvaRing - contraception, systemic)

IV. COMPARISON SUMMARY TABLE

Route	Bioavailability	Onset	First-Pass	Key Use
IV	100% (F = 1)	Seconds	None	Emergencies, precise control
IM	75-100%	Minutes	None	Depot drugs, vaccines
SC	75-100%	Minutes-hours	None	Insulin, heparin, implants
Intradermal	Low (intentional)	-	None	Allergy testing, vaccines
Oral	Variable (5-100%)	30-90 min	Yes	Routine outpatient therapy
Sublingual	High for lipophilic	1-5 min	None	Nitroglycerin, buprenorphine
Rectal	Partial (~50% bypasses)	15-30 min	Partial	Vomiting, unconscious patients
Transdermal	Variable	Hours	None	Prolonged systemic effect
Inhalation	Variable	Seconds-minutes	None	Respiratory disease, anaesthesia
Intrathecal	N/A (direct CNS)	Minutes	None	Spinal anaesthesia, CNS infections
Nasal	Good for small molecules	Minutes	None	Systemic/local nasal drugs

V. FIRST-PASS METABOLISM - KEY CONCEPT

The first-pass effect (presystemic elimination) is the metabolism of a drug in the gut wall and liver before it reaches systemic circulation. It is relevant to all routes where blood drains into the portal circulation (oral, rectal to a degree).

Routes that AVOID first-pass metabolism: IV > Inhalation > Sublingual/Buccal > Transdermal > Nasal > Rectal (partial) > IM/SC

Clinical implication: Drugs with high first-pass extraction (e.g., morphine, propranolol, GTN, lignocaine) need much smaller parenteral doses than oral doses. For example, oral morphine bioavailability is ~30% but IV is 100%.

Sources: Lippincott Illustrated Reviews: Pharmacology; Goodman & Gilman's The Pharmacological Basis of Therapeutics

Plasma half life shortly

Finding Sources

Reading File

I have the content needed. Here is a concise, clear answer:

Plasma Half-Life (t½)

Definition

The plasma half-life (t½) is the time required for the plasma concentration of a drug to fall to 50% of its current value.

For a first-order process:

t½ = 0.693 × Vd / CL

Where:

Vd = Volume of distribution
CL = Clearance
0.693 = ln 2 (natural log of 2)

Key Properties

Most drug elimination follows first-order kinetics - a constant fraction (not amount) is eliminated per unit time
Half-life is constant regardless of drug concentration (in first-order kinetics)
After 4-5 half-lives, a process (elimination or accumulation) is considered essentially complete (~97%)

Drug Elimination Over Time

Half-lives elapsed	% Drug eliminated	% Remaining
1	50%	50%
2	75%	25%
3	87.5%	12.5%
4	93.75%	6.25%
5	96.9%	~3%

Clinical Significance

Dosing interval: Drugs with short t½ need more frequent dosing; long t½ allows once-daily dosing
Time to steady state: Steady-state plasma concentration is reached after ~5 half-lives, regardless of dose or dosing frequency
Duration of action: A longer t½ generally means a longer duration of effect
Drug washout: After stopping a drug, it is effectively eliminated in ~5 half-lives (important before switching drugs, e.g., MAOIs)
Loading dose: Used when rapid steady state is needed for drugs with long t½ (e.g., amiodarone, digoxin)

Zero-Order Kinetics (Exception)

A few drugs (e.g., phenytoin, ethanol, aspirin at high doses) follow zero-order kinetics - a fixed amount is eliminated per unit time (enzyme saturation). In this case:

t½ is not constant - it increases as concentration rises
Small dose increases can cause disproportionately large rises in plasma levels → toxicity risk

Examples of Drug Half-Lives

Drug	t½	Clinical implication
Aspirin	15-20 min	Metabolite (salicylate) t½ is much longer
Penicillin G	~30 min	Requires frequent IV dosing
Gentamicin	2-3 h	Once-daily dosing used clinically
Digoxin	36-48 h	Takes ~7-10 days to reach steady state
Amiodarone	Weeks-months	Very prolonged effect; slow washout
Fluoxetine	1-4 days	No washout period needed before most drugs

Summary: t½ governs dosing frequency, time to steady state, and duration of drug effect. It is determined by both the volume of distribution and clearance - any disease or factor that increases Vd or reduces CL will prolong t½.

Source: Harrison's Principles of Internal Medicine, 22nd Edition

Drug tolerance

Finding Sources

Reading File

Finding Sources

Reading File

Excellent content. Here is the comprehensive answer:

Drug Tolerance

Definition

Drug tolerance is a state in which repeated administration of the same dose produces a diminishing effect, or conversely, increasing doses are required to produce the same effect. It represents a rightward shift of the dose-response curve.

"Tolerance is a rightward shift of the dose-response curve and is a state in which an increased dosage of a psychoactive substance is needed to produce a desired effect." - Barash's Clinical Anesthesia

Types of Tolerance

1. Innate (Natural) Tolerance

Genetically predetermined, pre-existing reduced sensitivity to a drug
Present before any drug exposure
Example: Certain individuals have naturally low sensitivity to alcohol due to genetic variants in alcohol dehydrogenase

2. Acquired Tolerance

Develops after repeated drug exposure. Has three subtypes:

A. Pharmacokinetic (Dispositional) Tolerance

Reduced drug concentration at the site of action due to altered drug metabolism or distribution
Most common mechanism: Enzyme induction of CYP450 system → accelerated hepatic metabolism → lower plasma drug levels for the same dose
Examples:
- Barbiturates induce their own metabolism (autoinduction) and that of other drugs
- Rifampicin induces CYP enzymes → reduced levels of co-administered drugs
- Carbamazepine undergoes autoinduction - its own t½ shortens after starting therapy
- Ethanol (chronic use) induces CYP2E1

B. Pharmacodynamic (Cellular) Tolerance

Drug concentration remains the same but the cellular/receptor response is diminished
Most important and clinically significant mechanism
Mechanisms include:

Mechanism	Description	Example
Receptor downregulation	Prolonged agonist exposure reduces receptor density (number)	β₂-agonists (salbutamol) → reduced bronchodilation
Receptor desensitization	Receptor uncoupled from effector (G protein) without loss in number	Opioid receptors uncoupled from Gi proteins
Receptor internalization	Agonist-bound receptors internalized (endocytosed) and either recycled or degraded in lysosomes	Opioid receptor trafficking via β-arrestin/clathrin pathway
Increased NMDA receptor sensitivity	Counterregulatory upregulation of excitatory receptors	Opioid tolerance - NMDA contributes to reduced analgesia
Physiological adaptation	Compensatory changes in downstream signaling; counter-regulation by opposing systems	Opioids → reflex sympathetic activation; nitrates → reflex renin-angiotensin activation

C. Learned (Behavioral) Tolerance

Compensatory behavioral adaptations mask the overt effects of the drug
The organism learns to function despite drug intoxication
Example: Experienced alcohol drinkers appear less intoxicated at the same blood alcohol level due to learned behavioral compensation

Tachyphylaxis

Rapid tolerance developing within minutes to hours of just a few doses
Due to rapid receptor desensitization or depletion of mediator stores
Examples:
- Ephedrine - acts by releasing noradrenaline from nerve terminals; stores become depleted rapidly
- Amphetamine - similar mechanism
- LSD - rapid tachyphylaxis within days; animals do not self-administer repeatedly
- Nitrates (short-acting) if used continuously

Cross-Tolerance

Tolerance to one drug confers tolerance to pharmacologically related drugs acting at the same receptor
Example: Morphine-tolerant patients show reduced response to all opioids (codeine, fentanyl, oxycodone)
Important: Cross-tolerance between opioids is incomplete - different drugs may have different degrees of tolerance at the same receptor. This is exploited clinically in opioid rotation (switching to a different opioid at >50% below the calculated equianalgesic dose to restore analgesia)

Differential Tolerance

Not all effects of a drug develop tolerance at the same rate:

Opioid Effect	Tolerance Development
Analgesia	Relatively faster
Sedation	Relatively faster
Nausea/vomiting	Relatively faster
Respiratory depression	Slower
Constipation	Slowest (often persists)
Miosis	Slowest (often persists)

Clinical implication: An opioid-tolerant patient who requires higher doses for pain relief may still have significant respiratory depression risk - the analgesic effect and respiratory depressant effect do not develop tolerance equally.

Tolerance vs Dependence vs Addiction

These are distinct concepts that are frequently confused:

Term	Definition
Tolerance	Rightward shift of dose-response curve; need more drug for same effect
Physical dependence	Physiological state of adaptation; withdrawal syndrome emerges on abrupt cessation or antagonist administration
Psychological dependence	Craving and compulsive drug-seeking behavior despite harm
Addiction	Chronic, treatable medical disease involving complex interactions of brain circuits, genetics, environment, and life experiences; compulsive use despite harmful consequences

"Tolerance is not synonymous with dependence. The development of tolerance or physical dependence in no way implies that the patient is addicted to an opioid." - Miller's Anesthesia

Opioid Withdrawal (Physical Dependence)

On abrupt cessation of opioids, withdrawal is characterized by:

Increased sympathetic activity: hypertension, tachycardia, diaphoresis
GI effects: abdominal cramping, diarrhoea, nausea
CNS: anxiety, restlessness, insomnia, yawning
Musculoskeletal: myalgia, piloerection ("goose flesh" - hence "cold turkey")

Clinical Examples of Drug Tolerance

Drug	Type of Tolerance	Mechanism	Clinical Impact
Opioids (morphine)	Pharmacodynamic	Receptor desensitization, downregulation, uncoupling from G proteins	Increasing dose needed for analgesia
Nitrates (GTN)	Pharmacodynamic	Tolerance develops within 24h of continuous use; involves free radical generation, reduced thiol groups, neurohormonal activation	Nitrate-free interval (8-12h/day) required
β₂-agonists (salbutamol)	Pharmacodynamic	β-arrestin-mediated receptor downregulation	Reduced bronchodilation with overuse
Benzodiazepines	Pharmacodynamic	GABA-A receptor uncoupling	Reduced anxiolytic/hypnotic effect
Barbiturates	Pharmacokinetic	CYP enzyme autoinduction	Shorter duration of effect
Ephedrine	Tachyphylaxis	Depletion of noradrenaline stores	Rapid loss of pressor effect with repeat doses
Alcohol	Pharmacokinetic + Learned	CYP2E1 induction + behavioral adaptation	Higher blood alcohol needed for same effect

Mechanism of Nitrate Tolerance (Special Note)

Organic nitrates (GTN, isosorbide dinitrate) develop tolerance rapidly during continuous use:

Mechanisms include: sulphydryl group depletion (needed for bioactivation), increased reactive oxygen species generation, reflex neurohormonal activation (renin-angiotensin, sympathetic), and plasma volume expansion
Prevention: A nitrate-free interval of 8-12 hours/day (typically overnight for angina patches)

Reversal of Tolerance

Rest (drug-free period): Receptor numbers recover; tolerance reverses
Opioid rotation: Use incomplete cross-tolerance to restore analgesic sensitivity
NMDA antagonists (e.g., ketamine): Can partially prevent or reverse opioid tolerance
Buprenorphine: As a partial opioid agonist, used in managing opioid dependence precisely because it limits tolerance development

Sources: Katzung's Basic and Clinical Pharmacology, 16th Ed; Barash, Cullen & Stoelting's Clinical Anesthesia, 9th Ed; Miller's Anesthesia, 10th Ed

This is a shared conversation. Sign in to Orris to start your own chat.