Trichomoniasis does not cause diarrhea. The parasite responsible for this infection, Trichomonas vaginalis, lives exclusively in the urogenital tract and does not infect the gut. However, the medications used to treat trichomoniasis are a well-known source of gastrointestinal side effects, including diarrhea, which is likely what many people experience and then attribute to the infection itself.
Why Trichomoniasis Doesn’t Affect the Gut
Trichomonas vaginalis inhabits the vagina in women, the prostate and seminal vesicles in men, and the urethra in both sexes. It is strictly a genitourinary parasite. It does not colonize the intestines, and stool tests for the organism come back negative even in people with confirmed urogenital infections. In one documented case where urine tested positive for T. vaginalis twice, stool analysis from the same patient was completely negative.
There is a related organism called Pentatrichomonas hominis that does live in the intestinal tract, specifically around the cecum (the beginning of the large intestine). Despite being a relative of T. vaginalis, this intestinal species is not considered pathogenic. There is no evidence it causes disease or diarrhea in humans. So even the trichomonad species that does live in the gut isn’t known to cause symptoms.
The Real Culprit: Treatment Side Effects
If you’re experiencing diarrhea around the time of a trichomoniasis diagnosis, the most likely explanation is the medication. The standard treatment is metronidazole, an antibiotic that is effective against the parasite but notorious for upsetting the stomach. The CDC-recommended regimen for women is a 500 mg dose taken twice daily for seven days. Men typically receive a single 2-gram dose. Both regimens commonly cause nausea, stomach cramps, and diarrhea.
Metronidazole works by disrupting the DNA of the parasite, but it also affects gut bacteria in the process. This disruption of normal intestinal flora is what triggers digestive symptoms for many people. The side effects tend to be worst with the higher single dose and usually resolve within a few days of finishing treatment.
An alternative medication, tinidazole, is sometimes prescribed instead. It reaches higher concentrations in the body, has a longer half-life (about 12.5 hours compared to 7.3 hours for metronidazole), and produces fewer gastrointestinal side effects. Diarrhea is still listed as a possible reaction to tinidazole, but it occurs less frequently. If stomach problems from metronidazole are severe, tinidazole may be worth discussing as an option. It does tend to cost more.
Other Reasons for Diarrhea During an Infection
Stress and anxiety around a new STI diagnosis can genuinely cause loose stools or upset stomach. The gut-brain connection is strong, and emotional distress is a recognized trigger for changes in bowel habits. If your diarrhea started before you began treatment, this is a plausible explanation.
It’s also possible to have a separate, unrelated gastrointestinal infection at the same time as trichomoniasis. Parasites like Giardia, bacterial infections, food intolerances, and viral gastroenteritis all cause diarrhea and have nothing to do with trichomoniasis. If diarrhea persists after you’ve finished your course of medication, or if it’s accompanied by fever, blood in the stool, or significant dehydration, a separate cause is worth investigating.
What Trichomoniasis Actually Feels Like
The symptoms of trichomoniasis are confined to the genital and urinary areas. In women, the most common signs include a frothy, yellow-green vaginal discharge with a strong odor, itching or irritation of the vulva, and discomfort during urination or sex. In men, symptoms are less common but can include irritation inside the penis, mild discharge, or a burning sensation after urination or ejaculation.
Up to 70% of people with trichomoniasis have no symptoms at all, which is part of why it spreads so easily. When symptoms do appear, they stay localized to the genitourinary tract. Digestive symptoms like diarrhea, bloating, or abdominal cramping are not part of the clinical picture of this infection. If you’re experiencing both genital symptoms and persistent diarrhea that can’t be explained by your medication, you may be dealing with two separate conditions that need individual attention.