Metronidazole is primarily used to treat trichomoniasis, one of the most common curable sexually transmitted infections worldwide. It’s also a key part of treatment for bacterial vaginosis (a condition closely linked to sexual activity) and pelvic inflammatory disease. If you’ve been prescribed metronidazole or are wondering why your provider chose it, here’s what it does and what to expect.
Trichomoniasis: The Main STI Target
Trichomoniasis is caused by a parasite called Trichomonas vaginalis, and metronidazole is the go-to treatment. The CDC recommends different regimens depending on sex: women typically take 500 mg twice a day for seven days, while men usually receive a single larger dose all at once. Women living with HIV follow the same seven-day course.
Metronidazole works by breaking down the DNA inside the parasite. The drug gets activated only in organisms that thrive without oxygen, which is exactly what Trichomonas is. Your own cells don’t activate the drug the same way, which is why it targets the infection without damaging healthy tissue.
Both sexual partners need treatment at the same time, even if only one person has symptoms. Without simultaneous treatment, the infection simply passes back and forth. Many people with trichomoniasis, especially men, have no symptoms at all, so a negative test in a partner doesn’t mean they’re clear.
Bacterial Vaginosis
Bacterial vaginosis (BV) isn’t classified strictly as an STI, but it’s strongly associated with sexual activity: having multiple partners, a new partner, or inconsistent condom use all raise the risk. BV happens when the normal balance of vaginal bacteria shifts, allowing certain organisms to overgrow. It also increases susceptibility to other STIs, including HIV, gonorrhea, chlamydia, and trichomoniasis itself.
Metronidazole treats BV either as a pill (500 mg twice daily for seven days) or as a vaginal gel applied once daily for five days. The oral and topical options have similar effectiveness, so your provider may let you choose based on preference. One important note: treating a male partner for BV has not been shown to prevent recurrence, so partner treatment isn’t currently recommended for this condition.
Pelvic Inflammatory Disease
Pelvic inflammatory disease (PID) is an infection of the uterus, fallopian tubes, or ovaries, often caused by STI-related bacteria spreading upward from the cervix. PID treatment requires multiple antibiotics because several types of bacteria are usually involved. Metronidazole is added specifically to cover anaerobic bacteria, organisms that grow in low-oxygen environments deep in the reproductive tract. These bacteria can cause serious tissue damage if left untreated.
For PID, metronidazole is taken alongside other antibiotics for a full 14 days. It’s part of the regimen whether treatment happens in the hospital with IV medications or at home with oral pills. The addition of metronidazole also helps clear any BV present at the same time, which is common in women with PID.
What Metronidazole Does Not Treat
Metronidazole has no effect on chlamydia, gonorrhea, syphilis, herpes, or HIV. These infections require entirely different medications. If you’re being tested for STIs, it’s common to be screened for several at once, and a positive result for trichomoniasis doesn’t rule out other infections. If your provider prescribed metronidazole alone, it means the infection they’re targeting is one that responds to this specific drug.
Common Side Effects
The most frequently reported side effect is a metallic taste in your mouth, which can persist throughout the course of treatment. Nausea, vomiting, stomach cramps, and diarrhea are also common. In clinical studies, roughly 10% of people stopped treatment early because of these gastrointestinal symptoms. The metallic taste is harmless but can be unpleasant enough to affect appetite. Eating bland foods or sucking on sugar-free candy can help.
Less common but more serious side effects include numbness or tingling in the hands and feet (a sign of nerve irritation) and, rarely, seizures. These typically occur with longer courses or higher doses and resolve after stopping the medication.
The Alcohol Rule
You need to avoid all alcohol while taking metronidazole and for at least 48 to 72 hours after your last dose. Mixing the two can trigger intense nausea, vomiting, stomach cramps, headaches, and facial flushing. This reaction is similar to what happens with medications designed to discourage drinking. Products containing propylene glycol, found in some medications, mouthwashes, and food items, should also be avoided during this window.
This isn’t a mild suggestion. The reaction can be severe enough to send people to the emergency room, and it’s one of the most well-established drug-alcohol interactions in medicine. Set a reminder for 72 hours after your final pill to be safe.
What to Do if Treatment Doesn’t Work
Most people clear their infection with a standard course of metronidazole. If symptoms persist after finishing treatment, the most common explanation is reinfection from an untreated partner rather than drug resistance. True metronidazole resistance in trichomoniasis exists but is uncommon. When it does occur, providers typically try a higher dose or a longer course of the same drug, or switch to a closely related medication. Retesting is generally recommended two weeks to three months after treatment to confirm the infection is gone.