The Journey of a Drug: An Overview of Absorption
Absorption is the process by which a drug moves from its site of administration into the bloodstream. This critical step in pharmacokinetics determines how much of the drug ultimately reaches its target. The site and speed of absorption are influenced by the route of administration, as each method presents different physiological barriers and conditions.
Oral Absorption: The Role of the Small Intestine
The most common route of administration is swallowing a tablet or capsule, which involves the gastrointestinal (GI) tract. While some weakly acidic drugs may be absorbed in the stomach, the majority of oral drug absorption happens in the small intestine. This is primarily due to the small intestine's extensive surface area, permeable membranes, and favorable pH. For further details on oral absorption, refer to {Link: Merck Manuals https://www.merckmanuals.com/professional/clinical-pharmacology/pharmacokinetics/drug-absorption}.
Routes That Bypass First-Pass Metabolism
Some routes bypass the liver's first-pass metabolism, ensuring a higher drug concentration reaches systemic circulation. Examples include sublingual, intravenous, transdermal, and inhalation routes. Intravenous administration provides 100% bioavailability. More information on these routes can be found on {Link: Merck Manuals https://www.merckmanuals.com/professional/clinical-pharmacology/pharmacokinetics/drug-absorption}.
Factors Influencing Drug Absorption
Drug absorption is influenced by both drug properties and patient characteristics. Drug factors include solubility and dissolution, lipid vs. water solubility, and pH and pKa. Patient factors include blood flow, gastric emptying time, presence of food, age, and health. For a detailed list of these factors, see {Link: Merck Manuals https://www.merckmanuals.com/professional/clinical-pharmacology/pharmacokinetics/drug-absorption}.
Comparing Major Absorption Pathways
| Route of Administration | Primary Absorption Site | Absorption Speed | Bypass First-Pass Metabolism? | Bioavailability (Approx.) |
|---|---|---|---|---|
| Oral | Small Intestine | Variable (Slow-Fast) | No (Subject to) | <100% (Highly Variable) |
| Sublingual/Buccal | Oral Mucosa | Rapid | Yes | Higher than oral |
| Intravenous | Systemic Circulation | Immediate | N/A (Directly Injected) | 100% |
| Intramuscular | Muscle Tissue | Intermediate | Yes (Mostly) | 60-100% |
| Transdermal | Skin (Dermis) | Slow (Sustained) | Yes | Variable, generally high |
| Inhalation | Alveoli (Lungs) | Very Rapid | Yes | Generally high |
| Rectal | Rectal Mucosa | Variable | Partially | Variable (Unpredictable) |
More comparative data on absorption pathways is available on {Link: Merck Manuals https://www.merckmanuals.com/professional/clinical-pharmacology/pharmacokinetics/drug-absorption}.
Conclusion
Understanding where are drugs usually absorbed is fundamental in pharmacology, impacting drug development and dosage. The choice of administration route, combined with drug properties and patient factors, is key to effective and safe medication delivery. For more information, refer to {Link: Merck Manuals https://www.merckmanuals.com/professional/clinical-pharmacology/pharmacokinetics/drug-absorption}.
