Expansion
Un-ionised drug is absorbed, so weak acids favour the stomach and weak bases the intestine
Expansion
Only the un-ionised form of a drug crosses lipid membranes.
- Weak acids (aspirin) are un-ionised in the acidic stomach, so in theory absorbed there
- Weak bases (opioids) are un-ionised in the alkaline intestine
In practice the small intestine absorbs the great majority of both, because its surface area of around 200 square metres and its rich blood flow overwhelm the pH effect. This is why gastric emptying rate is usually a more important determinant of the speed of absorption than pH.
Practical consequences of altered gastric pH:
- Proton pump inhibitors raise gastric pH and reduce absorption of drugs needing acid: ketoconazole, itraconazole, atazanavir, and iron salts
- They increase absorption of acid-labile drugs
- Calcium carbonate requires acid, while calcium citrate does not
Other absorption interactions worth pairing with this: chelation of tetracyclines and quinolones by calcium, iron and antacids, and altered absorption in coeliac disease or after bariatric surgery.