How does gastric acid vs. small intestine pH influence drug absorption?

The small intestine provides a more favorable environment for drug absorption due to its higher pH compared to the stomach. The stomach is highly acidic, which means that weak acids tend to be better absorbed there, but many drugs require a less acidic (more alkaline) environment to be absorbed effectively. The small intestine has a neutral to slightly alkaline pH, which helps in the dissolution and absorption of various drugs, including weak bases that would not be effectively absorbed in the acidic conditions of the stomach.

Additionally, the large surface area of the small intestine, facilitated by villi and microvilli, increases the chance for drug absorption significantly compared to the relatively smaller absorptive surface area of the stomach. This combination of a favorable pH and a greater absorptive surface makes the small intestine the primary site for drug absorption in the gastrointestinal tract.