Can an STD Cause Diarrhea?

Yes — several STDs and sexually transmitted gut infections can cause diarrhea, through distinct mechanisms. Rectal gonorrhea, chlamydia (including LGV serovars), herpes, and syphilis cause proctitis — rectal inflammation that mimics diarrhea; acute HIV infection causes watery diarrhea as part of the early illness syndrome; and Shigella flexneri, increasingly transmitted sexually in MSM, causes severe bloody diarrhea.

Proctitis: When Rectal STDs Mimic Diarrhea

The most common STD-related cause of diarrhea-like symptoms is proctitis — inflammation of the rectum — rather than true diarrhea from intestinal infection. Rectal gonorrhea (Neisseria gonorrhoeae), chlamydia (Chlamydia trachomatis), herpes (HSV-1 or HSV-2), and syphilis can all cause proctitis in people who have had receptive anal sex.

Proctitis symptoms: rectal pain, urgency, tenesmus (the sensation of needing to defecate when the rectum is empty or nearly so), rectal discharge (mucoid or bloody), and frequent small bowel movements that can be mistaken for diarrhea. The key distinction: proctitis produces frequent passage of small amounts of mucus, blood, or discharge rather than the large-volume watery diarrhea of intestinal infection. Testing requires a rectal swab NAAT — a standard urine test is entirely negative for rectal infection.

LGV: The Most Severe Proctitis

Lymphogranuloma venereum (LGV) — caused by Chlamydia trachomatis serovars L1, L2, and L3 — produces the most severe form of proctitis. Unlike standard chlamydia, LGV serovars invade lymphatic tissue and cause systemic inflammation. Rectal LGV presents with severe proctitis, bloody mucoid discharge, fever, and sometimes perianal ulceration. It can mimic inflammatory bowel disease closely enough to result in colonoscopy and IBD workup before the STD is considered.

LGV has been increasing in MSM populations in the US and Europe since the early 2000s. A standard chlamydia NAAT detects LGV (same species), but LGV confirmation testing is needed to identify the specific serovars and guide treatment duration — 21 days of doxycycline rather than the standard 7 days.

I have seen LGV diagnosed after months of IBD workup in patients who weren't asked about their sexual history. The proctitis is severe enough, and the presentation specific enough, that any sexually active patient with proctitis symptoms — especially MSM — should have rectal STD testing before gastrointestinal investigation proceeds.

HIV: Two Different Mechanisms

HIV causes diarrhea through two distinct mechanisms at different stages of infection. Acute HIV infection (the acute retroviral syndrome, occurring 2 to 4 weeks after exposure): watery diarrhea lasting 1 to 2 weeks, accompanied by fever, fatigue, pharyngitis, rash, and lymphadenopathy. This is the body's inflammatory response to primary viremia and resolves as the acute phase ends. Chronic HIV enteropathy: in untreated advanced HIV disease, the virus directly damages the intestinal mucosa and disrupts immune homeostasis in the gut, causing chronic diarrhea. With modern ART, HIV enteropathy is rare in treated patients.

Shigella: A Sexually Transmitted Gut Infection

Shigella flexneri is primarily a foodborne and waterborne pathogen, but in MSM populations, sexual transmission through oral-anal contact (rimming) or fecal-oral routes during sex is well-documented and increasingly significant. Shigellosis causes severe bloody diarrhea with cramping, fever, and tenesmus — symptoms that are more severe than most STD proctitis presentations. Shigella flexneri strains circulating in MSM networks are increasingly resistant to standard antibiotics including ampicillin and fluoroquinolones; treatment now often requires azithromycin or ceftriaxone based on susceptibility testing.

Shigella is not routinely tested in standard STD panels. It requires stool culture with sensitivity testing. Any MSM with severe bloody diarrhea should have stool culture alongside rectal STD swabs.

Giardia: Another Sexually Transmitted Gut Parasite

Giardia lamblia — a protozoan parasite causing profuse watery diarrhea, bloating, and malabsorption — is primarily waterborne but transmits sexually through fecal-oral contact, documented in MSM populations. It's not a classic STD but belongs to the category of sexually transmitted enteric infections. Diagnosis requires stool antigen test or PCR; treatment is metronidazole or tinidazole.

When Diarrhea After Sexual Exposure Warrants Testing

Diarrhea starting 2 to 4 weeks after a potential HIV exposure: consider acute HIV testing (4th generation antigen/antibody test). Rectal symptoms (urgency, tenesmus, mucoid discharge, rectal pain) after anal sex: rectal swabs for gonorrhea, chlamydia, herpes, and syphilis. Severe bloody diarrhea after oral-anal contact: stool culture for Shigella, Campylobacter, and enteric pathogens alongside STD testing. Standard STD panel urine tests will miss all rectal infections.

For STD testing including site-specific rectal swabs, Health Test Express offers panels with results in 1 to 2 days.

Frequently Asked Questions

What STD causes diarrhea most commonly?

Rectal gonorrhea and chlamydia cause proctitis — symptoms that mimic diarrhea — most commonly. LGV causes the most severe proctitis. Acute HIV infection causes true watery diarrhea as part of the early infection illness. Shigella, increasingly sexually transmitted in MSM, causes bloody diarrhea.

Can chlamydia cause diarrhea?

Rectal chlamydia causes proctitis with symptoms including urgency and frequent small bowel movements that resemble diarrhea. LGV serovars of chlamydia cause more severe rectal inflammation with bloody discharge. A urine chlamydia test misses rectal infection entirely — a rectal swab is required.

How do I know if diarrhea is from an STD or something else?

Rectal STD proctitis typically follows receptive anal sex and produces rectal pain, urgency, and mucoid discharge — rather than large-volume watery diarrhea. Acute HIV syndrome produces watery diarrhea with fever and rash 2 to 4 weeks after exposure. Shigella causes severe bloody diarrhea. Testing is the only way to distinguish STD-related from non-STD-related gastrointestinal illness.

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This article is for informational purposes only and does not constitute medical advice.