Herbal medicines are mixtures of more than one active ingredient. The multitude
of pharmacologically active compounds obviously increases the likelihood of
interactions taking place. Hence, the likelihood of herb–drug interactions is
theoretically higher than drug–drug interactions, if only because synthetic drugs
usually contain single chemical entities. Case reports and clinical studies have
highlighted the existence of a number of clinically important interactions, although
cause-and-effect relationships have not always been established. Herbs and drugs
may interact either pharmacokinetically or pharmacodynamically. Through
induction of cytochrome P450 enzymes and/or P-glycoprotein, some herbal products
(e.g. St John’s wort) have been shown to lower the plasma concentration (and/or the
pharmacological effect) of a number of conventional drugs, including cyclosporine,
indinavir, irinotecan, nevirapine, oral contraceptives and digoxin. The majority of
such interactions involves medicines that require regular monitoring of blood levels.
To date there is less evidence relating to the pharmacodynamic interaction. However,
for many of the interactions discussed here, the understanding of the mechanisms
involved is incomplete. Taking herbal agents may represent a potential risk to
patients under conventional pharmacotherapy.