Can STIs cause diarrhoea?

Sometimes, but diarrhoea isn’t a common STI symptom. It can be caused by:

• gut infections that can be passed on during sex, such as Shigella, Campylobacter and Giardia
• STIs that spread further into the bowel, such as LGV, a more invasive type of Chlamydia
• HIV, as long-lasting diarrhoea with no clear cause can be a sign of it, and some HIV medicines can cause diarrhoea

STIs that infect the rectum, such as Gonorrhoea and Chlamydia, often cause no symptoms. When they do, the usual signs are pain, discharge or bleeding from your bottom, rather than diarrhoea.

Most diarrhoea is caused by a stomach bug, such as norovirus, or by food poisoning. It usually stops within 5 to 7 days.

Other causes include:

• medicines, including antibiotics
• anxiety
• a food intolerance or food allergy
• irritable bowel syndrome (IBS)
• inflammatory bowel disease (IBD), such as Crohn’s disease and ulcerative colitis
• coeliac disease

Diarrhoea and vomiting can spread easily. Stay off work until you haven’t been sick or had diarrhoea for at least 2 days.

Shigella

Shigella is a gut infection that can be passed on during sex, particularly among gay, bisexual and other men who have sex with men. It spreads through mouth to bottom contact (rimming), giving oral sex after anal sex, and unwashed hands, and only a tiny amount of bacteria is needed to cause an infection.

Symptoms usually start 1 to 4 days after infection and include diarrhoea, which can be bloody, stomach cramps and a fever. In England, there were 2,560 cases of sexually transmitted Shigella in 2025, and more than half of the Shigella sonnei samples tested were resistant to all three of the usual first choice antibiotics. If you have Shigella, avoid sex until 7 days after your last symptom. Also avoid preparing food for other people, sharing towels, and using spas, swimming pools or hot tubs. You can stay infectious for up to a month, so keep washing your hands well.

Other gut infections

Campylobacter, Giardia and some types of E. coli can also be passed on through mouth to bottom contact. Campylobacter and E. coli can cause watery or bloody diarrhoea, stomach pain and a fever. Giardia usually causes watery or greasy, smelly diarrhoea, stomach cramps, bloating and wind. Hepatitis A spreads in the same way and can cause diarrhoea or constipation, as well as yellowing of the skin and eyes. Read more in “Can you get an STI from rimming?”

Inflammation of the rectum is called proctitis. It can cause pain in your bottom, a discharge, bleeding, or a feeling that you need to poo when you don’t. When it’s caused by an STI, the most common causes are Gonorrhoea, Chlamydia (including LGV), Herpes and Syphilis.

Gonorrhoea and Chlamydia

Gonorrhoea and Chlamydia can infect the rectum through anal sex or sharing sex toys. Chlamydia can also be found in the rectum of women who haven’t had anal sex. Most rectal infections cause no symptoms. When they do, the usual signs are pain, itching, discharge or bleeding from your bottom. Read more in “How do I know if I have an anal Gonorrhoea infection?”

LGV

Lymphogranuloma venereum (LGV) is a less common, more invasive type of Chlamydia. In the UK, it mostly affects gay, bisexual and other men who have sex with men. In England in 2024, 1,252 of the 1,274 LGV diagnoses where gender was recorded were in men. It can cause pain, bleeding, a discharge of mucus or blood, constipation or loose poo, and a feeling that you haven’t emptied your bowels.

Herpes

Herpes is a common cause of proctitis in men who have sex with men, often without any sores you can see around the bottom. Herpes spreading further into the bowel is rare. It mainly affects people with a weakened immune system, such as people taking medicines that lower the immune system for Crohn’s disease or ulcerative colitis. These medicines aren’t used to treat IBS.

Syphilis

Syphilis can infect the rectum and cause proctitis. It can also cause small sores (ulcers) around your bottom, which are usually painless.

Mpox

Mpox can cause pain or bleeding from your bottom, as well as a rash. The Better2Know Monkeypox Swab home test checks a swab taken from a spot or blister.

HIV attacks and destroys CD4 cells, a type of white blood cell that helps your body fight infections. Without treatment, this slowly weakens your immune system.

Some people get a short flu-like illness 2 to 6 weeks after catching HIV, usually with a sore throat, a high temperature, swollen glands, aches, tiredness or a rash. Diarrhoea can happen, but it isn’t one of the main early symptoms, and many people with HIV have no symptoms for years. Diarrhoea that lasts a long time with no clear cause is one of the reasons doctors offer an HIV test.

Without treatment, people with HIV are more likely to get gut infections that cause long-lasting diarrhoea. Some HIV medicines can also cause side effects such as diarrhoea, so tell the doctor or nurse who manages your HIV treatment if this happens. Read more in “How long does it take to show symptoms of HIV?”

Get urgent medical care if you have diarrhoea and:

• there is blood in your poo, or bleeding from your bottom
• it has lasted more than 7 days
• you have been sick for more than 2 days, or you keep being sick and can’t keep fluids down
• you still have signs of dehydration, such as dark, smelly pee or peeing less than usual, after using rehydration sachets
• you vomit blood, or your sick looks like ground coffee or is green
• you have a sudden, severe tummy ache or headache
• you have a stiff neck and pain when you look at bright lights
• your skin, lips or tongue look blue, grey, pale or blotchy
• you have severe difficulty breathing, or are taking lots of quick, short breaths
• you’re confused or not responding as usual

A doctor can test a sample of your poo to check for gut infections such as Shigella.

If you have pain, discharge or bleeding from your bottom, or you have anal sex, a urine test won’t pick up an infection in your rectum. You need a rectal swab, which you can usually take yourself. UK guidelines recommend that sexually active gay, bisexual and other men who have sex with men are tested at three sites: urine, throat and rectum.

• Chlamydia and Gonorrhoea (Rectal Swab): checks your rectum for Chlamydia and Gonorrhoea, from 14 days, with results in 2 days.
• Confidence Screen: tests a urine sample and throat and rectal swabs for Chlamydia and Gonorrhoea, and your blood for HIV, Syphilis and Hepatitis B, as well as Mycoplasma and Ureaplasma, from 28 days.
• HIV 4th Generation: a blood test for HIV, from 28 days. UK guidance gives a window period of 45 days for this type of test, so you may need a repeat test. It is also available as a home test kit.

These tests don’t check for gut infections such as Shigella. Not sure which test you need? Our Patient Care Team is available 24/7 on 020 7099 0955.

Can Chlamydia cause diarrhoea?

Not usually. Chlamydia in the rectum often causes no symptoms, or pain, discharge or bleeding from your bottom. LGV, a less common type of Chlamydia, can sometimes cause loose poo.

Is diarrhoea an early sign of HIV?

It can happen, but it isn’t one of the main early symptoms. Only some people get an early flu-like illness, usually with a sore throat, a high temperature, swollen glands, aches or a rash, and many people have no symptoms for years. If you might have been at risk, get tested whether or not you have symptoms. Long-lasting diarrhoea with no clear cause is also a reason to get tested.

Can you catch a stomach bug from sex?

Yes. Gut infections such as Shigella, Campylobacter and Giardia can be passed on through mouth to bottom contact, or by giving oral sex after anal sex. Washing your hands and using a dental dam for rimming lower the risk.

Diarrhoea is usually caused by a stomach bug or food poisoning. But some infections passed on during sex, such as Shigella, can cause it, and STIs in the bottom can cause pain, discharge or bleeding, or no symptoms at all.

If you have symptoms in your bottom or you have anal sex, get tested. Book a Confidence Screen or a Chlamydia and Gonorrhoea (Rectal Swab) test at a clinic near you. Our Patient Care Team is available 24/7 on 020 7099 0955.

Not in the UK? Choose your country from the selector at the top of the page for local tests and prices.

This article originally appeared on 11 October 2024 and was last updated on 3 October 2026.

Medical review. This article was medically reviewed by Dr Steve Chapman, Doctor with a wealth of clinical experience in the field of STIs, on 3 December 2024. It is general information and not a substitute for personal medical advice.

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