Bioavailability of digoxin: some pitfalls and problems.

The bioavailability of tablet formulations averages about 60% for digoxin, 75% for beta-acetyldigoxin, and 75% for beta-methyldigoxin. Bioavailability as a measure of the absolute amount reaching the systemic circulation should be calculated from steady state data. Only in steady state does the retarding effect of absorption disappear which diminishes the p.o./i.v. relation of data. For a screening test bioavailability may be calculated from 24-hour renal excretion values after a single dose, performing absolute and relative studies consecutively in cross-over arrangements. The absorption rate constant of digoxin and its derivatives is approximately 0.7 (h-1) which corresponds to an absorption half life of about 1 hour. The absorption rate constant can be calculated from plasma concentration values as well as from renal excretion rates 1 or 2 hours after dosing.