What STD Causes Stomach Pain and Diarrhea?

Several sexually transmitted infections can cause stomach pain and diarrhea, so there is no single STD responsible. The most common culprits include chlamydia, gonorrhea, syphilis, herpes simplex virus, and sexually transmitted intestinal parasites like Entamoeba histolytica. How they produce gut symptoms varies: some directly infect the rectum or colon, others climb into the pelvic organs and radiate pain into the abdomen, and a few can even attack the stomach lining itself. The overlap with conditions like inflammatory bowel disease makes these infections easy to miss, which is part of why they deserve a closer look.

Chlamydia and Gonorrhea as Gut Infections

When people think of chlamydia and gonorrhea, they usually picture genital infections. But both organisms readily infect the rectum and lower colon after receptive anal sex, producing a condition called infectious proctitis. The most frequently identified agents in proctitis cases are Neisseria gonorrhoeae (gonorrhea), Chlamydia trachomatis, herpes simplex virus, and Treponema pallidum (syphilis).¹ Symptoms include rectal pain, bloody discharge, cramping, and an urgent feeling of needing to use the bathroom even when the bowel is empty. When the infection extends higher into the sigmoid colon, it becomes proctocolitis, and that is when diarrhea and more generalized abdominal cramping become prominent.

A particularly aggressive form of chlamydia deserves special attention. Certain strains of Chlamydia trachomatis cause lymphogranuloma venereum (LGV), which goes beyond simple proctitis. LGV can produce rectal bleeding, pain, and a sensation of incomplete evacuation that closely resembles proctocolitis and is sometimes mistaken for a rectal malignancy.² Cases of LGV have been increasing among men who have sex with men in Europe and North America, and clinicians sometimes do not think to test for it because the presentation so closely mimics inflammatory bowel disease on colonoscopy.³ If you are experiencing persistent rectal symptoms and have had unprotected anal sex, raising the possibility of LGV with your doctor can avoid months of misdiagnosis.

Syphilis in the Stomach and Colon

Syphilis has been called “the great imitator” for centuries, and its ability to affect the gastrointestinal tract is a prime example of why. While most people associate syphilis with genital sores and skin rashes, Treponema pallidum can directly infect the stomach lining. In one documented case, a 33-year-old man presented with three weeks of upper abdominal pain, nausea, vomiting, and significant weight loss before gastric biopsies revealed Treponema pallidum organisms in his stomach tissue.⁴ That presentation is strikingly similar to a gastric ulcer or even stomach cancer, which is exactly what makes gastric syphilis dangerous: it gets worked up as something else entirely.

Syphilis does not stop at the stomach. A review of lower gastrointestinal syphilis found that common symptoms included abdominal pain, diarrhea, constipation, mucous discharge, and a persistent urge to defecate.⁵ When the infection settles in the rectum or colon, it can produce colitis severe enough to warrant biopsies and repeated visits to gastroenterology before anyone thinks to order a syphilis blood test. This is especially true in patients who do not have an obvious genital chancre or rash at the time their gut symptoms appear.

Herpes Simplex Virus Proctitis

Herpes simplex virus, usually type 2, is another common cause of rectal inflammation after sexual transmission. In a study of over 100 sexually active men presenting with anorectal symptoms, acute herpes was detected at a significantly higher rate than in men without gastrointestinal complaints. Men with HSV proctitis reported severe anorectal pain in every case, along with tenesmus, and nearly half had fever or difficulty urinating.⁶ Sacral nerve tingling or numbness occurred in about a quarter of cases, which is an unusual clue that can help distinguish herpes proctitis from other causes.

The symptoms can be dramatic enough to trigger a full colonoscopy. A case report described a teenager who presented with pain during bowel movements, bloody rectal discharge, and diarrhea; the initial clinical picture looked consistent with inflammatory bowel disease, but rectal biopsies grew herpes simplex virus.⁷ The good news is that once HSV proctitis is correctly identified, antiviral treatment with drugs like valacyclovir tends to resolve the acute episode fairly quickly. The bad news is that, like genital herpes, the virus can reactivate and cause recurrent bouts of rectal symptoms.

Sexually Transmitted Parasites

Gut parasites are not always acquired from contaminated food or travel to tropical regions. Amebiasis and giardiasis are two parasitic infections commonly transmitted through sexual contact, particularly through oral-anal exposure.⁸ Entamoeba histolytica causes amoebic dysentery, with bloody diarrhea, cramping, and lower abdominal pain. Giardia intestinalis causes a different pattern: watery, foul-smelling diarrhea, bloating, and nausea, usually affecting the small intestine rather than the colon.

A cross-sectional study of men who have sex with men presenting with gastrointestinal symptoms at an STI clinic in Barcelona found Entamoeba histolytica in about one in five patients, making it the most common pathogen in that group. Giardia intestinalis was detected in roughly one in nine patients. Entamoeba was most strongly associated with proctocolitis symptoms (lower abdominal pain and bloody diarrhea), while Giardia was linked to upper GI enteritis (watery diarrhea, bloating, nausea).⁹ If you have persistent diarrhea and have engaged in oral-anal contact, a stool parasite test is worth requesting, because standard STI panels do not screen for these organisms.

Shigella and Other Enteric Bacteria

Shigella is a bacterial infection traditionally associated with contaminated food and poor sanitation, but it has increasingly been recognized as a sexually transmitted pathogen. Even a tiny number of organisms can cause infection, which is why person-to-person spread during sex, especially involving any oral-anal contact, is efficient. Shigella produces classic dysentery symptoms: watery or bloody diarrhea, abdominal cramps, and fever. Outbreaks among men who have sex with men have been documented repeatedly, including an outbreak of extensively drug-resistant Shigella sonnei in the UK. Interviews with patients during that outbreak revealed limited awareness among both patients and their healthcare providers that Shigella could be sexually transmitted.¹⁰

Drug resistance adds a worrying dimension. Some Shigella strains circulating in sexual networks have become resistant to most first-line antibiotics, making treatment more complicated and increasing the risk of prolonged illness. Genomic surveillance has been used to track these transmission networks, linking whole genome sequencing data with behavioral and clinical information to map how Shigella flexneri spreads among men who have sex with men.¹¹ The practical takeaway: if you develop sudden-onset bloody diarrhea and have had recent sexual contact involving oral-anal exposure, tell your doctor explicitly so they can order the right culture and susceptibility testing.

When Pelvic Infections Cause Abdominal Pain

Not all STD-related stomach pain comes from a direct gut infection. In women, untreated chlamydia or gonorrhea can ascend from the cervix into the uterus and fallopian tubes, causing pelvic inflammatory disease. PID produces lower abdominal pain and pelvic tenderness, and patients often describe it as stomach pain because the sensation radiates across the lower abdomen.¹² Some people also experience nausea or changes in bowel habits alongside the pelvic symptoms.

Mycoplasma genitalium is a less well-known sexually transmitted bacterium that is increasingly recognized as a cause of PID independent of chlamydia and gonorrhea. Studies have linked Mycoplasma genitalium to a substantially elevated risk of endometritis, and European guidelines note that in women, the infection is associated with PID in roughly 10 to 25 percent of cases, with symptoms that include abdominal pain and painful intercourse.¹³¹⁴ Because Mycoplasma genitalium is not included in most routine STI screening panels, it often goes undiagnosed until a patient presents with unexplained pelvic or abdominal pain.

In rare cases, PID-causing organisms spread beyond the pelvis entirely. Fitz-Hugh-Curtis syndrome is a type of perihepatitis in which bacteria infect the capsule around the liver, producing sharp right upper quadrant pain that can be mistaken for gallbladder disease or even a heart-related problem with right-sided chest pain.¹⁵ It sometimes occurs without any obvious signs of pelvic infection, which makes it even more confusing for clinicians. One case report described a young woman admitted to the emergency department with right upper quadrant pain, fever, elevated liver function tests, and vaginal discharge; the constellation of findings eventually pointed to Fitz-Hugh-Curtis syndrome rather than the gallbladder problem initially suspected.¹⁶

Acute HIV Infection

The initial phase of HIV infection, sometimes called acute retroviral syndrome, can include gastrointestinal symptoms. A study of adults diagnosed during the acute phase of HIV found that diarrhea occurred in about a third of cases.¹⁷ The diarrhea typically appears alongside a flu-like illness two to four weeks after exposure, with fever, sore throat, muscle aches, and sometimes a rash. Because those symptoms overlap with countless other infections, acute HIV rarely gets diagnosed at this stage unless a provider thinks to test for it. If you develop an unexplained illness with diarrhea and abdominal discomfort after a potential exposure, requesting an HIV test (specifically a fourth-generation antigen/antibody test, which picks up early infection) is reasonable.

How These Infections Get Mistaken for IBD

One of the more serious clinical pitfalls with sexually transmitted gut infections is how closely they resemble inflammatory bowel disease. Multiple STI-related infections produce the same endoscopic appearance — redness, ulceration, mucosal swelling — that clinicians associate with Crohn’s disease or ulcerative colitis. A study comparing STI-related colitis with IBD identified several features that helped distinguish the two: a history of being an HIV-positive man who has sex with men, anal pain, and anal discharge were statistically significant predictors of STI colitis rather than IBD. Tissue biopsies in STI colitis generally showed only mild crypt damage and lacked the eosinophil-rich inflammation typical of IBD.¹⁸

The danger of misdiagnosis runs both ways. If an STI is mistaken for IBD, a patient may be started on immunosuppressive drugs that make the underlying infection worse. Conversely, if a flare of actual IBD is assumed to be an STI because the patient has risk factors, the real condition goes untreated. The overlap is especially tricky with LGV proctosigmoiditis, which can produce tissue changes on biopsy that are essentially indistinguishable from Crohn’s disease without specific chlamydia testing. Any new diagnosis of rectal or sigmoid inflammatory disease in a sexually active person warrants STI screening before immunosuppressive therapy begins.

How Sexual Transmission of Gut Pathogens Happens

For organisms like Shigella, Giardia, and Entamoeba, the key transmission route is oral-anal contact, either directly or indirectly through fingers or shared objects. This is the fecal-oral route operating during sex. A study of men who have sex with men found that anilingus (rimming) carried the highest odds of acquiring sexually transmitted enteric pathogens.¹⁹ Indirect oral-anal contact through activities like fingering was also identified as a transmission route.²⁰

For chlamydia, gonorrhea, syphilis, and herpes, the transmission route is typically direct mucosal contact during anal sex. These organisms infect the rectal lining on contact. Syphilis can additionally spread through oral sex and then disseminate systemically to reach the stomach and other organs via the bloodstream, which is how gastric syphilis develops even in people who have never had anal intercourse. Understanding the specific route matters because it shapes prevention: dental dams and gloves reduce fecal-oral transmission during oral-anal contact, while condoms reduce mucosal-contact transmission during anal sex. Neither method is commonly discussed in sexual health education despite the clear link between these practices and gut infections.

Long-Term Gut Effects After the Infection Clears

Even after an enteric infection is successfully treated, some people develop lasting changes in bowel function. Roughly ten percent of people with irritable bowel syndrome can trace the start of their symptoms to a previous episode of infectious dysentery, a phenomenon called post-infectious IBS.²¹ The infection itself resolves, but the gut’s nervous system and immune responses remain altered, leaving behind chronic abdominal pain, bloating, and irregular bowel habits that can persist for months or years. While this has been studied mostly in the context of foodborne infections like Salmonella and Campylobacter, there is no biological reason why sexually transmitted enteric infections like Shigella or amoebic dysentery would not carry the same risk. If you develop lingering gut symptoms long after an STI affecting your digestive tract has been treated and confirmed cleared, post-infectious IBS is worth discussing with your gastroenterologist.

Why Providers Often Miss the Connection

A recurring theme across the research is that healthcare providers frequently fail to consider STIs as a cause of gastrointestinal symptoms. Part of the problem is compartmentalized medicine: GI symptoms go to the gastroenterologist, genital symptoms go to the sexual health clinic, and the two rarely talk to each other. A patient presenting with bloody diarrhea to an emergency room or GI clinic is far more likely to be evaluated for IBD, infections acquired from food, or even colorectal cancer than for an STI. The UK Shigella outbreak report specifically noted that both patients and their healthcare providers showed limited understanding of Shigella as a sexually transmitted pathogen.¹⁰

The same pattern holds for syphilis, herpes, and LGV. These infections are well documented as causes of abdominal pain and diarrhea in the medical literature, yet they remain underappreciated in routine clinical practice. Sexual history, particularly detailed questions about specific practices like receptive anal sex and oral-anal contact, is the single most useful screening tool. If your provider does not ask and you suspect a connection, volunteering the information yourself can redirect the workup toward the right tests and avoid unnecessary procedures like repeated colonoscopies or empiric immunosuppressive therapy for a condition you do not actually have.