Absorption and excretion of weak acids and bases
- "anti-ulcer" drugs can significantly alter the effectiveness of other drugs by altering either absorption or elimination
- the drugs most affected are weak acids and weak bases, which can more easily cross membranes in their non-polar forms
- non-polar forms are more easily absorbed from the stomach
- ionized forms are more readily excreted by the kidneys
- as outlined in the table below, raising pH (whether in the stomach or in the renal tubule) will
- shift the equilibrium of weak acids to the right (ionized form)
- shift the equilibrium of weak bases to the left (non-polar form)
GASTRIC ABSORPTION |
EXCRETION |
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Low pH (<2) NORMAL |
Low pH (<8*) NORMAL |
High pH (>8) systemically absorbed antacids |
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Weak acids HA ↔ H+ + A- |
more |
decrease |
less |
increase |
Weak bases BOH ↔ B+ + OH- |
less |
increase |
more |
decrease |
* urine pH normally varies between 4 and 8; the exact effect on excretion will depend on the pKa of the specifc drug |
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