Metronidazole

Metronidazole targets organisms that live without oxygen — anaerobic bacteria and protozoa. Its cleverness is selectivity: the drug is a prodrug that only becomes toxic inside anaerobic cells, where their metabolism reduces it into reactive radicals that shred the microbe’s DNA. Human cells and aerobic bacteria don’t reduce it, so they’re spared.

That gives it a specific niche: intra-abdominal and pelvic infections, bacterial vaginosis, trichomoniasis, giardiasis, amebiasis, part of H. pylori regimens, and Clostridioides difficile colitis (though oral vancomycin/fidaxomicin are now generally preferred for C. diff).

The signature teaching point is the disulfiram-like reaction: patients are told to avoid all alcohol during therapy and for ~3 days after (72 h; tinidazole longer). Drinking is said to cause flushing, throbbing headache, nausea/vomiting, and palpitations. (Honesty note: newer evidence questions whether this reaction is real/consistent, but the warning remains on the label and in standard nursing teaching, so the safe advice is still to abstain — and to check other products, like some mouthwashes, for alcohol.)

Other effects: a metallic taste and dark/reddish-brown urine (harmless), GI upset, and — with prolonged/high-dose use — peripheral neuropathy (numbness/tingling) and rare CNS effects. Metronidazole also potentiates warfarin, raising bleeding risk.