Absorption of a drug occurs primarily in which site?

• A. Stomach
• B. Liver
• C. Small intestine
• D. Large intestine

Answer: (C)

The main idea is that most drug absorption happens where there is the greatest surface area and good blood flow to allow the drug to cross into the bloodstream. The small intestine fits that role perfectly because its lining has enormous surface area thanks to folds, villi, and microvilli, plus it’s well perfused and the transit time is sufficient for absorption of many drugs. This combination lets lipophilic drugs diffuse across the mucosa efficiently, which is why the small intestine is the primary site.

The stomach, while it can absorb some substances, has a relatively small absorptive surface and a harsh acidic environment that isn’t ideal for most drugs. The large intestine mainly reabsorbs water and electrolytes and offers less surface area for drug absorption, so its contribution is limited. The liver is not a site of absorption; after crossing the intestinal wall, absorbed drugs enter the portal circulation and may undergo metabolism there (first-pass effect) before reaching systemic circulation.