Common drug–drug interactions between prescription medications, non-prescription medications, and complementary therapies

Overview

Non-prescription medications (over-the-counter drugs, complementary and alternative medicines, herbal supplements) can cause clinically significant interactions with prescribed medications, but are frequently not volunteered by patients unless specifically and directly asked, given a common patient perception that "natural" or non-prescription products don't count as "real" medications worth mentioning
A significant, under-recognised source of drug interactions given high population rates of complementary/alternative medicine and over-the-counter product use, often concurrent with prescribed medication without the prescriber's knowledge

Herbal/complementary products can cause pharmacokinetic interactions (e.g. St John's Wort's potent cytochrome P450 induction reducing levels of numerous co-prescribed drugs, including oral contraceptives and some antiretrovirals/immunosuppressants) or pharmacodynamic interactions (e.g. combining multiple serotonergic supplements/OTC products with a prescribed antidepressant, increasing serotonin syndrome risk)

Structured, explicit enquiry specifically naming complementary/alternative medicines and over-the-counter products (not just asking generically "are you on any medications") substantially improves disclosure rates, given patients' tendency not to volunteer this information unprompted
See for the detailed medication history-taking approach

Explicitly reviewing complementary/alternative medicine and OTC use before prescribing any new medication with known interaction potential
Patient education that "natural" products can still interact with prescribed medications, addressing the misconception that only prescription drugs warrant disclosure

St John's Wort and oral contraceptive/antiretroviral/immunosuppressant interaction (a frequently tested, clinically significant example), NSAID and warfarin/antihypertensive interactions from OTC use, serotonergic supplement and antidepressant interaction risk
St John's Wort, despite being a widely available "natural" supplement, is a potent cytochrome P450 enzyme inducer causing clinically significant reductions in the efficacy of numerous prescribed medications (including oral contraceptives, some antiretrovirals, and immunosuppressants) - patients frequently do not disclose its use unless specifically asked, given the common misconception that herbal products don't count as "real" medication

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