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Pharmacology - Definition

"The subject of pharmacology is a broad one and embraces the knowledge of the source, physical and chemical properties, compounding, physiological actions, absorption, fate, and excretion, and therapeutic uses of drugs. A drug may be broadly defined as any chemical agent that affects living protoplasm, and few substances would escape inclusion by this definition."

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The Two Core Branches

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Scope of the Field

1. Source - Where drugs come from (plants, animals, synthesis, biotechnology)
2. Physical & chemical properties - Solubility, molecular weight, stability
3. Physiological actions - How drugs alter normal body function
4. Absorption, fate & excretion - How the body handles the drug over time
5. Therapeutic uses - Clinical applications in treating disease
6. Toxicology - Harmful effects, dosing thresholds, poisoning
7. Pharmacogenetics - How genetic variation affects drug response
8. Drug development & regulation - From discovery through clinical trials to approval

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Related Sub-disciplines

• Clinical pharmacology - Drug use in human patients
• Neuropharmacology - Drug effects on the nervous system
• Psychopharmacology - Drugs affecting mood, cognition, behavior
• Pharmacovigilance - Post-market drug safety monitoring
• Molecular pharmacology - Drug-receptor interactions at the molecular level

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Drugs - Definition & Classification

What is a Drug?

"A drug may be broadly defined as any chemical agent that affects living protoplasm, and few substances would escape inclusion by this definition."

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Classification of Drugs

1. By Source / Origin

"Today, only a fraction of new drugs are discovered in nature. Most drugs are not discovered but are totally new compounds, painstakingly optimized... In that sense, today's new drugs are more invented than discovered." - Goodman & Gilman

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2. By Therapeutic Use (Pharmacotherapeutic Class)

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3. By Mechanism of Action

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4. By Legal/Regulatory Status

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5. By Chemical Nature

• Small molecules - most traditional oral drugs (aspirin, statins, antibiotics)
• Biologics - large protein-based molecules (monoclonal antibodies, hormones, vaccines)
• Nucleic acid-based - siRNA, mRNA therapies (e.g. mRNA COVID vaccines)

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Key Properties That Define a Drug

1. Efficacy - ability to produce the intended effect
2. Potency - the dose required to produce the effect
3. Selectivity - how specifically it acts on its target vs. other tissues
4. Safety/Toxicity - the therapeutic window (gap between effective and toxic doses)
5. Pharmacokinetics - how it is absorbed, distributed, metabolized, and excreted (ADME)

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Drug Dosage Forms

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Overview of Routes of Administration

"The route of administration is determined by properties of the drug (e.g., water or lipid solubility, ionization) and by the therapeutic objectives (e.g., need for rapid onset, long-term treatment, or restriction to a local site)." - Lippincott

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1. ENTERAL (via the GI tract)

A. Oral (PO) - Most Common Route

• Bioavailability: 5% to <100% (oral) - Katzung's Basic and Clinical Pharmacology

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B. Oral Liquid Forms

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C. Sublingual (SL) / Buccal

• Sublingual: Drug placed under the tongue
• Buccal: Drug placed between cheek and gum
• Both bypass the GI tract and avoid first-pass metabolism
• Rapid absorption directly into bloodstream
• Examples: Glyceryl trinitrate (GTN) for angina, buprenorphine

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D. Rectal (PR)

• Drug delivered as suppositories, enemas, or gels
• ~50% bypasses the portal circulation, reducing first-pass metabolism
• Useful when: patient is vomiting, unconscious, or cannot take oral drugs
• Bioavailability: 30 to <100%
• Absorption can be erratic and incomplete
• Examples: Diazepam rectal solution, paracetamol suppositories

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2. PARENTERAL (injected - bypasses GI tract)

• Depot injection - Slow-release drug in oil or microparticles (e.g., long-acting antipsychotics, contraceptive implants)
• Liposomal/lipid complex - Drug encapsulated in liposomes for targeted delivery (e.g., liposomal amphotericin B)

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3. INHALATION

• Drug delivered as aerosol, gas, dry powder, or nebulised liquid
• Rapid onset (nearly as fast as IV) due to large lung surface area
• Ideal for respiratory conditions (asthma, COPD) - drug acts directly at site of action, minimising systemic side effects
• Bioavailability: 5 to <100%
• Examples: Salbutamol inhaler (MDI), budesonide (DPI), anaesthetic gases (isoflurane, nitrous oxide)

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4. TOPICAL & TRANSDERMAL

"The rate of absorption [of transdermal drugs] can vary markedly, depending on the physical characteristics of the skin at the site of application, as well as the lipid solubility of the drug." - Lippincott

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5. NASAL & OCULAR

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Summary Comparison Table

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