Abstract
The bioavailability of a drug is usually assessed in healthy subjects. However, it is reasonable to expect that significant alterations in bioavailability may occur in actual patients with different diseases or in individuals belonging to special populations. Relatively few studies have been conducted to examine this possibility. The stable isotope method is well suited to compare absolute bioavailability in patients and healthy subjects. Studies in which this method was used indicate that significant changes in the bioavailability of some drugs are particularly likely in patients with advanced liver disease and in those whose splanchnic blood flow is reduced. The expectation is that bioavailability in neonates, children, and pregnant women may also differ from that in non-pregnant adults.