Metronidazole is the first-line treatment for trichomoniasis, the most common curable STD in the world. It’s also the go-to drug for bacterial vaginosis (BV) and plays a supporting role in treating pelvic inflammatory disease (PID). While metronidazole is prescribed for many types of infections throughout the body, these are the sexually transmitted and reproductive infections where it’s most commonly used.
Trichomoniasis: The Primary STD It Treats
Trichomoniasis is caused by a parasite called Trichomonas vaginalis, and metronidazole is the CDC-recommended treatment. The drug works by entering the parasite’s cells and damaging their DNA, which kills the organism. It’s one of only two medications approved for trichomoniasis in the U.S., the other being tinidazole, a closely related drug.
The treatment regimen differs by sex. For women, the standard course is 500 mg taken twice daily for seven days. For men, it’s typically a single 2-gram dose taken all at once. Women living with HIV follow the same seven-day regimen. The reason for the difference: studies have shown the multi-day course is more effective in women, likely because the parasite can persist in harder-to-reach areas of the reproductive tract.
One important detail: the vaginal gel form of metronidazole does not work well enough for trichomoniasis. It doesn’t reach high enough concentrations in the urethra and surrounding glands to clear the infection. Only the oral tablet form is recommended.
If the infection comes back after treatment, what happens next depends on whether you’ve been re-exposed to an untreated partner. Re-exposure means repeating the original regimen. If there’s no re-exposure, the dose is escalated to 2 grams daily for seven days. This is why partner treatment matters so much: without it, reinfection is almost inevitable.
Bacterial Vaginosis
BV isn’t technically classified as an STD, but it’s closely linked to sexual activity and is one of the most common reasons metronidazole gets prescribed. BV happens when the normal balance of bacteria in the vagina shifts, allowing certain anaerobic bacteria to overgrow. Metronidazole is effective here because it specifically targets these oxygen-avoiding bacteria while leaving healthy bacteria largely intact.
For BV, you have two options. The oral route is 500 mg twice a day for seven days, identical to the trichomoniasis regimen. The topical route is a 0.75% vaginal gel applied once daily for five days. Both are considered first-line treatments. A newer single-dose vaginal gel (1.3% concentration) is also available for those who prefer a one-time application.
Recurrent BV is frustratingly common, and metronidazole plays a role in long-term management as well. For women who keep getting reinfected, using the vaginal gel or a suppository twice weekly for three months or longer can help prevent recurrences. Some treatment plans combine an initial oral course with weeks of follow-up vaginal therapy to keep the infection from bouncing back.
Its Role in Pelvic Inflammatory Disease
PID is a serious infection of the uterus, fallopian tubes, or ovaries, often caused by bacteria that spread upward from the cervix. It’s frequently triggered by chlamydia or gonorrhea, but anaerobic bacteria are commonly involved too. Metronidazole is not the primary antibiotic for PID, but it’s included in nearly every recommended treatment combination because of its ability to eliminate those anaerobic organisms.
In PID treatment, metronidazole is paired with other antibiotics that cover chlamydia and gonorrhea. For outpatient treatment, the typical combination includes an injection to cover gonorrhea, a two-week course of doxycycline for chlamydia, and metronidazole 500 mg twice daily for 14 days to handle the anaerobic component. Adding metronidazole also effectively clears any BV that’s present alongside PID, which is a frequent overlap.
What Metronidazole Does Not Treat
Metronidazole has no activity against chlamydia, gonorrhea, syphilis, herpes, or HIV. These infections are caused by different types of organisms that metronidazole can’t reach or kill. Chlamydia and gonorrhea are bacterial, but they’re the wrong kind of bacteria for this drug. Herpes and HIV are viral, and no antibiotic works against them. If you’ve been prescribed metronidazole alone and are concerned about other STDs, those require separate testing and different treatments.
It’s also worth noting that metronidazole doesn’t treat yeast infections. In fact, because it kills off competing bacteria, it can sometimes allow yeast to overgrow, which is why some people develop a yeast infection during or after a course of treatment.
Side Effects to Expect
Metronidazole is generally well tolerated, but it has some distinctive side effects. The most recognizable is a metallic taste in the mouth, reported by roughly 43% of people taking it. About 35% notice a drop in appetite, and around 18% experience nausea or vomiting. Some people feel dizzy or lightheaded (about 23% in studies). These side effects are more noticeable with the oral tablets than the vaginal gel.
The most well-known precaution is avoiding alcohol while taking metronidazole. The combination has long been thought to cause a reaction similar to what happens with certain alcohol-aversion drugs: flushing, nausea, rapid heartbeat, and vomiting. Recent reviews suggest this reaction may be less predictable than once believed, varying in both frequency and severity, but the standard recommendation is still to avoid alcohol during treatment and for at least 24 to 48 hours after finishing your last dose.
Safety During Pregnancy
Metronidazole can be used during pregnancy when an infection needs to be treated. Large-scale data show no overall increased risk of birth defects from metronidazole exposure, and no clear evidence of increased risk of preterm delivery, low birth weight, or complications for the newborn. Some studies have noted an association with miscarriage in early pregnancy, though the underlying infection itself may independently raise that risk, making it hard to separate the drug’s effect from the disease’s.
The one adjustment during pregnancy is that high single-dose regimens are generally avoided in favor of the standard multi-day course. Exposure to metronidazole at any point in pregnancy is not considered grounds for additional fetal monitoring or concern about the pregnancy’s viability.