A client taking isoniazid reports acid reflux. What pharmacokinetic issue should you teach about?

A client taking isoniazid reports acid reflux. What pharmacokinetic issue should you teach about?

• A. Increased absorption of isoniazid
• B. Decreased absorption of isoniazid
• C. No change in absorption
• D. Variable absorption

Answer: (B)

Gastric pH affects how well a weakly basic drug is absorbed. Isoniazid acts like a weak base, so when the stomach becomes more acidic—as with acid reflux—the drug gets protonated and turns more ionized. The ionized form crosses cell membranes poorly, so less drug is absorbed from the gut into the bloodstream. That’s why the key pharmacokinetic issue to teach is decreased absorption in an acidic environment. If the stomach were less acidic, absorption could be relatively better for this drug.

Gastric pH affects how well a weakly basic drug is absorbed. Isoniazid acts like a weak base, so when the stomach becomes more acidic—as with acid reflux—the drug gets protonated and turns more ionized. The ionized form crosses cell membranes poorly, so less drug is absorbed from the gut into the bloodstream. That’s why the key pharmacokinetic issue to teach is decreased absorption in an acidic environment. If the stomach were less acidic, absorption could be relatively better for this drug.