Contraceptive efficacy may be affected by both changes in pharmacokinetics and
pharmacodynamics of hormonal contraceptives. Pharmacokinetic interactions occur when
one drug alters the absorption, distribution, metabolism or excretion of another, thereby
increasing or decreasing its serum concentration and its effects.
To have a clinical effect (e.g. inhibition of ovulation or thickening of cervical mucus) there
needs to be sufficient amount of hormone available at the site of action. Bioavailability of
contraceptive hormones depends primarily on absorption (including secondary absorption
via the enterohepatic circulation) and metabolism (Figure 1). Therefore, drugs that reduce the
absorption, metabolism or excretion of hormones may affect their bioavailability and
potentially affect contraceptive efficacy.
Pharmacodynamic interactions occur when one drug directly influences the clinical actions
of another by synergy or antagonism. For example, contraceptive steroids might reduce the
efficacy of antihypertensives, lipid-lowering drugs and antidiabetics because they can have
opposing actions.