If you’ve ever had a sudden bout of diarrhea a day or two after sex, your first thought was probably food poisoning. Sometimes it is. But sometimes the cause is an infection that was passed on during sex, and many people, and even some doctors, never think to ask.
The short answer: yes, an STI can cause diarrhea. Doctors don’t use the phrase “sexually transmitted diarrhea disease” as a formal diagnosis. They talk about sexually transmitted enteric (gut) infections. This guide explains which infections are involved, what the symptoms look like, how they’re tested and treated, and how to lower your risk.
What Is Sexually Transmitted Diarrhea Disease?
It’s an everyday way of describing gut infections that spread through sexual contact. The germs involved normally live in stool. They can pass from the stool or unclean fingers of one person into the mouth of another during sex, through oral or anal sex and anal play such as rimming and fingering, or indirectly through sex toys, used condoms and douching equipment.
That’s why it isn’t only about intercourse. The CDC notes that risk is present for any sexual activity involving stool exposure.
Which Infections Can Cause It?
Several different germs can be passed on this way. A European treatment guideline groups them by the part of the gut they affect:
| Where it hits | Infections |
|---|---|
| Rectum (proctitis) | Gonorrhea, chlamydia (including LGV), syphilis; herpes can also be involved |
| Rectum and colon (proctocolitis) | Shigella, Campylobacter, Salmonella, E. coli, Entamoeba histolytica |
| Small intestine (enteritis) | Giardia, Cryptosporidium, hepatitis A virus |
Of these, three stand out. A strong body of research describes sexual transmission of Campylobacter, Giardia lamblia and Shigella, especially antibiotic-resistant Shigella, in sexual networks of men who have sex with men.
Why It Matters
Most cases clear up. But there are a few reasons to take it seriously:
- It’s easy to miss. Specialists point out that many men who have sex with men don’t realize these infections can be sexually transmitted.
- Antibiotic resistance is growing. A CDC report found that the vast majority of US Shigella infections from 2011 through 2023 were in adult men, and some isolates were resistant to five oral antibiotics. Some of those isolates were still treatable with a few other drugs, such as oral carbapenems, pivmecillinam and fosfomycin.
- You can spread it without knowing. Shigella can be shed in stool for up to six weeks.
Symptoms: What to Look For
Symptoms depend on the germ. Shigella is the best-known, and the CDC says symptoms usually start one to two days after infection and last about a week. They include:
- Diarrhea that can be bloody or prolonged (lasting more than three days)
- Fever
- Stomach pain
- A feeling of needing to pass stool even when the bowels are empty
Rectal infections (proctitis) can add rectal pain, discharge or bleeding. Giardia and some other infections tend to cause more watery diarrhea, gas and cramps. Hepatitis A can bring fatigue, nausea and yellowing of the skin or eyes.
Diarrhea after unprotected sex doesn’t automatically mean an STI. Food, travel and other infections are far more common causes. The clues that point toward a sexual cause are timing (a day or two after a new or risky encounter), blood in the stool, and rectal pain or discharge.
Who Should Be Especially Careful?
Anyone who has sex can be affected, but some people have a higher risk:
- Gay, bisexual and other men who have sex with men. Shigella can spread easily and rapidly during sexual activity in this group. Risk rises with the number of sexual partners.
- People living with HIV or with weakened immune systems. They may get sicker or have longer illness.
- Anyone whose sex involves oral-anal contact, anal play, shared toys or douching equipment.
How Is It Diagnosed? (STI Test for Diarrhea)
Be open with your doctor about your sexual history. It changes which tests make sense. For adults with suspected Shigella, the CDC authors advise clinicians to culture the stool, take a sex history, discuss prevention, and choose treatment based on antibiotic susceptibility. Depending on your symptoms, your doctor may also offer:
- Stool testing for bacteria and parasites
- Rectal swab testing for gonorrhea and chlamydia (including LGV)
- Blood tests for syphilis, HIV and hepatitis, as appropriate
Doctors are encouraged to consider that enteritis or proctocolitis in men who have sex with men may have been sexually transmitted, and to arrange targeted STI testing.
Treatment: What to Expect
Treatment depends entirely on which germ is responsible, which is why testing matters.
- Shigella: Many people get better without antibiotics in five to seven days. The ECDC notes that treatment shortens symptoms by one to two days and may reduce the chance of passing it on. The choice of drug depends on resistance.
- Giardia: Options include tinidazole, nitazoxanide or metronidazole.
- Amoebiasis (Entamoeba histolytica): Treating both symptomatic and symptom-free infections is recommended.
- Hepatitis A: Treatment is supportive care.
- Rectal gonorrhea or chlamydia: treated with specific antibiotics based on test results.
Whatever the cause, drink plenty of fluids. The CDC advises fluids to prevent dehydration. Don’t take leftover antibiotics, and ask a doctor or pharmacist before using anti-diarrhea medicine, especially if your stool is bloody or you have a fever.
Prevention: How to Lower Your Risk
No method is perfect, but these steps help:
- Wash up. The CDC recommends washing hands, genitals and the anal area with soap and water before and after sex, and washing sex toys the same way.
- Use barriers. Condoms and dental dams add protection.
- Avoid sex when someone is ill. The California Department of Public Health advises avoiding sex with anyone who has had diarrhea in the past two weeks.
- Don’t share equipment. Avoid sharing douching materials, enemas and sex toys.
- Get vaccinated. Hepatitis A and B vaccination is offered to people who aren’t immune.
- Get regular sexual health checkups, since many infections cause no symptoms.
If you’re diagnosed, protect your partners too. The CDC says to avoid sex until at least two weeks after the diarrhea ends. The ECDC is more cautious about oral-anal contact: at least seven days without sex after symptoms fully disappear, and no oral-anal contact for four to six weeks. Ask your doctor which applies to you, and tell recent partners so they can get tested.
Common Mistakes to Avoid
- Assuming it’s just food poisoning and never mentioning sex to your doctor.
- Self-treating with old antibiotics, which can add to resistance.
- Resuming sex too soon, while you can still pass it on.
- Skipping the hepatitis A and B vaccines.
- Staying quiet, which leaves partners unaware.
When to See a Doctor
See a doctor promptly if you have:
- Bloody diarrhea
- Fever
- Severe stomach pain
- Diarrhea lasting more than three days
- Rectal pain, discharge or bleeding
- Signs of dehydration, such as dizziness, very little urine or extreme thirst
Go to urgent care if you can’t keep fluids down or feel very unwell. If you’re pregnant, immunocompromised or caring for a young child, don’t wait.
Frequently Asked Questions
Can an STI cause diarrhea?
Yes. Several infections spread through sex can cause diarrhea, including Shigella, Campylobacter, Giardia and Entamoeba. Rectal infections such as gonorrhea and chlamydia can also cause bowel symptoms.
Is Shigella an STI?
It isn’t classed as a traditional STI, but it can spread during sex, especially through oral-anal contact, anal sex and shared toys.
What does diarrhea after unprotected sex mean?
It could be many things, including food, travel or a stomach virus. If it started a day or two after a risky encounter, or comes with blood, fever or rectal pain, get tested.
What should I do about bloody diarrhea after sex?
See a doctor soon. Bloody diarrhea can signal a bacterial infection that needs testing and sometimes treatment.
How long am I contagious?
It varies by infection. Shigella can stay in stool for several weeks after symptoms end, so follow your doctor’s advice about when to resume sex.
Which STI test do I need for diarrhea?
There isn’t a single test. Your doctor may order stool tests, rectal swabs for gonorrhea and chlamydia, and blood tests for syphilis, HIV and hepatitis.
Can it be cured?
Many of these infections clear up on their own or are cured with the right treatment. Resistant strains can be harder to treat, so see a doctor.
Can women get these infections?
Yes. The CDC notes the risk applies to any sexual activity involving stool exposure, regardless of gender.
The Bottom Line
Diarrhea after sex isn’t something to be embarrassed about, and it isn’t always an STI. But if you have bloody diarrhea, fever, rectal symptoms, or a risky exposure, get tested and be honest with your doctor. Washing up, using barriers, getting vaccinated and avoiding sex while ill or recovering go a long way toward protecting you and your partners.
Related reading: Salmonella: Everything You Need to Know
Sources
- CDC: About Shigella Infection
- CDC: Shigella Infection Among Gay, Bisexual, and Other Men Who Have Sex with Men
- ECDC: Spread of multidrug-resistant Shigella in EU/EEA
- CIDRAP: Extensively drug-resistant Shigella on the rise in the US
- Clinical Infectious Diseases: Enteric Infections in Men Who Have Sex With Men
- 2021 European Guideline on proctitis, proctocolitis and enteritis
- CDC: Sexually Transmitted Infections Treatment Guidelines, 2021
This article is for general information only and is not medical advice. See our Medical Disclaimer.
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