Which factor affects drug bioavailability?

• A. Absorption
• B. Gastric pH
• C. Genetics
• D. Exercise

Answer: (A)

Bioavailability is the fraction of an administered dose that reaches the systemic circulation. The factor that most directly determines this is absorption—the process of moving the drug from its site of administration into the blood. If absorption is poor, only a small portion enters circulation, so bioavailability is low regardless of the dose.

Gastric pH can influence absorption for drugs whose solubility or ionization depends on acidity, so it can modify bioavailability in certain cases but isn’t the universal determinant. Genetics can affect enzymes and transporters that act after absorption, altering metabolism and distribution and thus systemic levels, but they don’t change the amount that enters circulation as directly as absorption does. Exercise generally does not have a direct effect on the amount of drug that reaches systemic circulation.