Metronidazole (brand name Flagyl) is the first-line treatment for bacterial vaginosis, and it's one of the most prescribed antibiotics in the U.S. Here's what side effects to expect, what's harmless, what's serious, and the one interaction you absolutely cannot ignore.
Metronidazole is a nitroimidazole antibiotic that's been around since the 1960s. It works by entering anaerobic bacteria (and some parasites), where bacterial enzymes convert it into a reactive compound that damages bacterial DNA. It's highly targeted at anaerobes — the organisms that cause bacterial vaginosis, trichomoniasis, and some abdominal infections — which is why it's on the CDC's first-line BV regimen.
The reason metronidazole has such distinctive side effects — metallic taste, the famous alcohol reaction, occasional dark urine — is that the drug gets distributed widely through body fluids (saliva, urine, vaginal secretions, CSF) and metabolized in a way that interferes with alcohol breakdown. Understanding that mechanism makes the side effects easier to anticipate.
Affects roughly 10% of patients. Happens because metronidazole is secreted into saliva and contacts the taste receptors on your tongue. Brushing your tongue, sugar-free hard candies, and taking the pill with food can dull it. It resolves within a few days of finishing the course. Not dangerous.
One of the most common reasons people discontinue early. Taking metronidazole with food — not on an empty stomach — reduces nausea significantly. Ginger, mint tea, or a bland snack before each dose helps. If you vomit within 30 minutes of a dose, you may need to re-dose; if it happens more than once, call the prescribing provider.
Usually mild and resolves with the course. Adequate hydration helps. Severe headache with confusion, trouble walking, or vision changes is a different entity — stop the drug and call a provider.
Metronidazole produces a pigmented metabolite that can turn urine a reddish-brown or darker-than-usual color. Per the FDA label, this is benign and resolves after the course ends. Dark urine combined with yellow eyes, right-upper-quadrant pain, or severe fatigue is different — that combination can signal liver stress and needs urgent attention.
Mild and common. Sugar-free gum, sips of water, and a humidifier help.
This one catches patients off guard: you're treating BV and end up with a yeast infection. It happens in roughly 10 to 15 percent of women treated with metronidazole, per AAFP guidance. The reason is mechanistic — metronidazole wipes out the anaerobes causing BV, but the vaginal ecosystem is complex, and Candida yeast can overgrow in the space left behind. If you develop thick white cottage-cheese discharge and intense itching during or just after a BV course, it's usually yeast. A single-dose oral fluconazole or an OTC miconazole typically clears it.
Less common but possible — white patches in the mouth that don't wipe off. Treatable with nystatin rinse or oral fluconazole.
Food can taste strange or bland during treatment. This is related to the saliva-secretion effect above. It resolves after the course.
Numbness, tingling, or burning pain in the hands or feet. This is most often seen with prolonged or repeated courses of metronidazole (months of treatment, as sometimes used for Crohn's or resistant infections) — rare with a standard 7-day BV course. If you develop numbness or tingling during treatment, stop the drug and call a provider. Usually reversible after stopping but can take weeks to months to improve fully, per NIH MedlinePlus.
Very rare. Presents as confusion, unsteady walking, slurred speech, or in the most extreme cases seizure. If this happens, it's an emergency — stop the medication and get evaluated. Almost always resolves after stopping the drug.
Hives, facial or throat swelling, difficulty breathing — the same presentation as any drug allergy. Rare but requires immediate 911 or ER attention.
Very rare with a standard course. Signs include yellowing of the eyes or skin, severe fatigue, right-upper-quadrant pain, or extremely dark urine with those other symptoms. Stop the drug and call a provider.
Metronidazole for BV comes in two main forms: a 500 mg oral tablet twice daily for 7 days, or a 0.75% vaginal gel applied once daily for 5 days. Per the FDA DailyMed label, systemic drug levels from the vaginal gel are only about 2 percent of oral levels.
Oral tablets: Higher rate of nausea, metallic taste, headache, and dark urine. Full systemic exposure means full alcohol-reaction risk.
Vaginal gel: Much less GI upset and systemic side effects. Most common complaints are local — vaginal irritation, itching, or a mild yeast overgrowth. The alcohol precaution is still recommended in CDC guidance because some systemic absorption does occur.
Both are equally effective at clearing BV in non-pregnant adults. The choice is often about tolerability — people who get severe nausea from one oral course often prefer the gel on a recurrence.
Flagyl is the original brand name; generic metronidazole is chemically identical and FDA-approved to the same bioequivalence standards. There is no clinical difference. Generic metronidazole is dramatically cheaper — a full 7-day course often costs under $10 with a GoodRx coupon. Unless your prescriber specifies otherwise, the generic is the right choice every time.
Stop the medication and get in touch with a prescriber for any of the following:
Most common side effects clear within 1 to 3 days after your last dose. Yeast overgrowth may need an antifungal. Peripheral neuropathy, if it occurred, usually improves over weeks to months.
Yes. There's no meaningful interaction between metronidazole and acetaminophen (Tylenol) or ibuprofen (Advil). Both are safe for headache or general discomfort.
No. Unlike some older antibiotics, metronidazole does not reduce the effectiveness of combined hormonal contraceptives or progestin-only pills. You do not need a backup method for this antibiotic alone.
You can have sex during BV treatment, but it may delay resolution of symptoms and increase irritation. Condom use is recommended because semen can alter vaginal pH. Partner treatment is not routinely required — male partners don't carry BV.