Chlamydia: what it is, symptoms, testing and treatment

Chlamydia is a sexually transmitted infection (STI) caused by a bacterium. It is cured with antibiotics, but it often causes no symptoms, so the only way to know whether you have it is to get tested.

It is one of the most common STIs. The World Health Organization (WHO) estimates there were 128.5 million new infections worldwide among adults aged 15 to 49 in 2020. In Europe, the European Centre for Disease Prevention and Control (ECDC) recorded 213,443 cases in 2024, making it the most frequently reported bacterial STI. Having it says nothing about who you are. Anyone who is sexually active can get it.

What it is

It is caused by the bacterium Chlamydia trachomatis. It can infect the urethra, the cervix, the rectum, the throat and, less often, the eyes. There is no vaccine against it.

Symptoms and when they appear

Most people notice nothing at all. When symptoms do appear, they can start anywhere from about a week to several months after contact. The most common are:

• Burning or discomfort when you pee.
• Unusual vaginal discharge, or a white, cloudy or watery discharge from the penis.
• Bleeding after sex or between periods.
• Pain in the lower tummy, or pain and swelling in the testicles.
• Pain, discharge or bleeding from the bottom, if the rectum is infected.
• Red, painful, weeping eyes, if it reaches the eyes.

In the throat it rarely causes symptoms. Feeling fine does not rule the infection out.

How it spreads – and how it doesn't

It spreads through vaginal, anal or oral sex without a condom with someone who has the infection. It can also pass on through sharing sex toys that have not been washed or covered with a new condom. During birth, it can pass from mother to baby.

It does not spread through hugging, sharing cups or cutlery, using the same toilet or towels, or swimming in the same pool. Because many people do not know they have it, it can be passed on without anyone realising.

Testing: when and how

The test is simple and painless. It is usually a urine sample or a swab from the vagina, penis, rectum or throat, depending on the kind of sex you have had. You can often take the swab yourself, and some services offer home testing kits.

After a risk, the infection can take up to two weeks to show up on a test. If you test earlier, you may be asked to repeat it. If you have symptoms, do not wait: get checked now.

It is worth testing with every new partner and, if your partners change, at least once a year. In the United States, the CDC recommends yearly screening for all sexually active women under 25. Find out more in how often to get tested for STIs.

Treatment: it can be cured

Chlamydia is cured with antibiotic tablets, either as a single dose or as a course lasting a few days. Treatment works very well and is sometimes started before results come back.

• When to have sex again: wait until you and your partners have finished treatment. The CDC advises waiting 7 days after a single dose, or until a 7-day course is finished and any symptoms have gone.
• Partners: current and recent partners also need testing and treatment. Otherwise the infection comes back.
• Retesting: reinfection is common. The CDC recommends testing again about three months after treatment, even if your partner was treated too.

Having had chlamydia does not protect you: you can catch it again.

Complications if it isn't treated

Without treatment, the infection can spread to the womb, fallopian tubes and ovaries and cause pelvic inflammatory disease. This can lead to long-term pelvic pain and raise the risk of ectopic pregnancy and infertility. In the testicles it can cause painful inflammation. Some people develop reactive arthritis, with painful joints. Untreated chlamydia also raises the risk of getting or passing on HIV.

Early treatment almost always prevents these problems. Damage that has already happened, however, may not be reversible.

Chlamydia and pregnancy

Untreated, it can increase the chance of premature birth and low birth weight. It can also pass to the baby during birth and cause an eye infection (conjunctivitis) or pneumonia. According to WHO, testing and treatment in pregnancy prevent the baby from getting it. If you are treated while pregnant, you will be offered a follow-up test about four weeks later. Read more in our pregnancy and postnatal guide.

Prevention

• Use a condom for vaginal and anal sex, and a condom or dental dam for oral sex. Check the common condom mistakes.
• Do not share sex toys or, if you do, wash them and put a new condom on them.
• Get tested regularly, especially with new partners.
• Always finish your treatment and tell your partners, so the infection is not passed back and forth.

When to get help

Book an appointment with your GP or a sexual health clinic if you have symptoms, a partner tells you they have chlamydia or another STI, you have had sex without a condom with a new partner, or you are pregnant or trying to conceive. Seek help more urgently if you have severe tummy pain, a fever, or pain and swelling in a testicle.

Your visit is confidential. It is a good chance to test for other infections too, such as gonorrhoea, which often appears alongside chlamydia, and HIV.