As contrasted with the sluggish digitoxin, digoxin is regarded as a glycoside that is uniformly absorbed and eliminated with considerable rapidity. Recent reports, however, disclose a frequent lack of equivalence in digoxin plasma levels, inconsistencies in its elimination rates, and discrepancies between the latter and its chronotropic effects.
Discussing bioavailability problems of digoxin, Wagner and associates (p 199) point out that in addition to individual tablet-to-tablet variations in potency due to poor mixing in manufacture, there are substantial potency differences between brands of digoxin. A two-way crossover study in eight subjects, demonstrated a 41% difference between peak plasma levels attained with tablets belonging to two well-known brands. This difference correlated well with the in vitro rate of dissolution of the tablets and thus could be easily predicted.
Less predictable than the lack of equivalence in bioavailability of digoxin are the inconsistencies in its elimination rates. In a study of five