Pharmacokinetics: Absorption, Metabolism, and Excretion Issues

Pharmacodynamics and Pharmacokinetics : Pharmacokinetics: Absorption, Metabolism, and Excretion Issues

The following generalizations can be made about absorption, metabolism, and excretion:

• Sex differences may exist in drug absorption and first-pass elimination. (The oral route of administration has been studied the most.)
• Sex differences can occur in systemic drug metabolism.
• Age-related differences in drug metabolism can be sex-specific.
• The sex-specific effects of drugs of the same pharmacologic class and similar structure may be different.

Sex-Specific Aspects of Drug Absorption

Route of Administration. While several factors potentially could cause a sex difference in drug absorption, no consistent sex differences have been identified and only a small number of sex differences in drug absorption have been documented. In some cases, the rate of drug absorption seems to be different for men and women and to depend on the route of drug administration. For example:

• Aspirin taken orally has been found to be absorbed more rapidly in women than in men, but reports on sex differences in the subsequent bioavailability of aspirin are inconsistent. In contrast, aspirin administered intramuscularly is absorbed more slowly in women than in men. Several explanations for this apparent sex difference have been offered and include sex differences in blood flow and possible injection into fat tissue rather than muscle in the women.⁷
• In a study of aerosolized ribavirin, deposits in the respiratory tract were significantly less for adolescent and older women than for men.⁸ This sex effect may be attributable to differences in breathing characteristics, or to physiological differences.
• Drugs that are applied vaginally accumulate to a greater extent in the uterus than when administered by other routes.⁹