Presentations associated with adverse drug interactions

At a glance

• An interaction is a cause to look for in any new problem after a medicine change. Most present within days to weeks of starting, stopping or changing a dose.
• Two mechanisms explain nearly all of them: pharmacokinetic (one drug changes the level of another) and pharmacodynamic (two drugs push the same effect).
• Risk concentrates in older people, CKD, polypharmacy, acute illness, and drugs with a narrow therapeutic index (warfarin, digoxin, lithium, tacrolimus, methotrexate, phenytoin, colchicine).