Examples of absorption in pharmakinetics
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FDA drug absorption routes administration pharmacokinetics overview
Absorption in pharmacokinetics is the movement of a drug from its site of administration into the systemic circulation. It is one part of ADME: absorption, distribution, metabolism, and excretion.
Examples
| Route | Example | How absorption occurs |
|---|---|---|
| Oral | Paracetamol, amoxicillin | Drug dissolves in the gastrointestinal tract and is absorbed mainly through the small intestine. |
| Sublingual/buccal | Nitroglycerin tablet | Absorbed through oral mucosa directly into blood, producing rapid action and avoiding first-pass liver metabolism. |
| Rectal | Diazepam rectal gel, paracetamol suppository | Absorbed through rectal mucosa; partly avoids first-pass metabolism. |
| Intramuscular | Vaccines, benzathine penicillin | Drug diffuses from muscle into capillaries. Blood flow and formulation affect rate. |
| Subcutaneous | Insulin, heparin | Drug moves from subcutaneous tissue into capillaries or lymphatics. |
| Inhalational | Salbutamol inhaler, inhaled anesthetics | Drug is absorbed across the large alveolar surface of the lungs. |
| Transdermal | Nicotine patch, fentanyl patch | Drug slowly crosses intact skin, giving sustained systemic delivery. |
| Intranasal | Naloxone nasal spray, desmopressin | Absorbed across the vascular nasal mucosa, often with rapid onset. |
| Topical local effect | Hydrocortisone cream | Usually intended to act locally in skin, though some systemic absorption can occur. |
Important exception
- Intravenous injection: there is no absorption step, because the drug is placed directly into systemic circulation. Bioavailability is 100%.
Factors that alter oral drug absorption
- Food: e.g., food can reduce or delay absorption of some medicines.
- Gastric pH: antacids or proton-pump inhibitors can alter absorption of pH-dependent drugs.
- Intestinal motility: diarrhea may reduce contact time and absorption.
- Drug interactions: calcium can bind tetracycline or levothyroxine and reduce their absorption.
- First-pass metabolism: oral propranolol, nitroglycerin, and morphine may lose some drug in gut wall/liver before reaching systemic circulation.
Absorption is assessed by its rate such as time to peak concentration (Tmax), and extent, often reflected by bioavailability and area under the concentration-time curve (AUC). The FDA likewise defines absorption as drug reaching systemic circulation and notes food, pH-altering medicines, intestinal enzymes, and transporters as influencing factors in its clinical pharmacology overview.
Factors affecting absorption of drugs
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FDA clinical pharmacology drug absorption factors food pH transporter first pass
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Factors affecting drug absorption can be grouped into drug-related, formulation-related, physiologic, and clinical factors.
| Factor | Effect on absorption | Example |
|---|---|---|
| Route of administration | Determines whether and how quickly a drug must cross a membrane. | IV drugs enter blood directly, so there is no absorption phase. Sublingual and inhaled drugs are generally absorbed rapidly. |
| Lipid solubility | More lipid-soluble drugs cross cell membranes more easily. | Lipid-soluble drugs tend to be absorbed well by passive diffusion. |
| Molecular size | Smaller molecules cross membranes more readily than large molecules. | Large peptides and proteins such as insulin are not effectively absorbed orally. |
| Degree of ionization and pH | The nonionized form crosses lipid membranes best. Drug absorption depends on drug pKa and local pH. | Weak acids such as aspirin are relatively more nonionized in acidic gastric fluid; weak bases are relatively more nonionized in the alkaline intestine. |
| Surface area available | Greater surface area increases absorption. | The small intestine is the major site of absorption because it has villi and microvilli. |
| Blood flow to the site | Higher blood flow maintains a concentration gradient and increases uptake. | IM absorption increases with exercise or a warm compress; it may decrease in shock. |
| Contact time | More contact between drug and absorbing surface generally increases absorption. | Diarrhea can reduce oral drug absorption; delayed gastric emptying can delay onset. |
| Gastric emptying and intestinal motility | Usually affects the rate at which an oral drug reaches the small intestine. | Metoclopramide may speed absorption of some drugs by increasing gastric emptying. |
| Food | May delay, reduce, or occasionally increase absorption. | Calcium-containing food can reduce tetracycline absorption by chelation. A fatty meal may increase absorption of some fat-soluble drugs. |
| Drug formulation | Disintegration, dissolution, particle size, salt form, coating, and excipients affect availability for absorption. | Enteric-coated tablets delay release until the intestine; sustained-release forms slow absorption. |
| Drug concentration and dose | A larger concentration gradient can increase passive diffusion, until saturation occurs. | Carrier-mediated transport may become saturated at higher doses. |
| Drug interactions | Other drugs can bind the drug, alter pH or motility, or affect transporters. | Antacids/PPIs alter pH-dependent absorption; cholestyramine can bind several drugs; P-glycoprotein can pump drugs back into the intestinal lumen. |
| First-pass metabolism | Gut-wall and hepatic metabolism reduce the amount of active drug reaching systemic blood after oral administration. | Nitroglycerin undergoes extensive first-pass metabolism, so it is commonly given sublingually or transdermally. |
| Disease and patient factors | GI disease, vomiting, diarrhea, malabsorption, poor perfusion, age, and surgery can alter absorption. | Celiac disease, bowel resection, heart failure with gut edema, or shock can reduce oral absorption. |
Quick memory aid
“Drug, dosage form, site, patient, and interactions.”
For oral drugs, the most exam-relevant factors are:
- Drug dissolution and lipid solubility
- pH and ionization
- Gastric emptying and intestinal motility
- Surface area and intestinal blood flow
- Food and drug interactions
- First-pass metabolism
- GI disease or malabsorption
Bioavailability is the rate and extent to which administered drug reaches systemic circulation. Oral drugs may have reduced bioavailability because of incomplete absorption and first-pass metabolism. Lippincott Illustrated Reviews: Pharmacology, pp. 40-41.
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