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Chlamydia is a sexually transmitted infection (STI) caused by Chlamydia trachomatis bacteria. It is the most common bacterial STI in the UK.

The infection is highest among sexually active individuals between the ages 15 and 24 years and often affects the vagina, penis, anus or testicles.

Chlamydia is transmitted through unprotected intercourse (vaginal, oral, or anal), contact with an infected person’s genitals, sharing unclean unprotected sex toys or infected semen or vaginal fluid getting into the eye.

It can also be transmitted from mothers to their unborn baby in the womb or during childbirth, causing conjunctivitis (an eye infection) which can lead to blindness.

Chlamydia, if not treated appropriately, can lead to serious health complications including:

  • Pelvic Inflammatory Disease (PID) an infection of the womb, fallopian tubes and ovaries
  • ectopic pregnancy (pregnancy outside of the womb)
  • infertility
  • premature birth
  • increased chances of catching HIV


Symptoms usually appear up to 3 weeks after exposure to the infection, but in some cases could be many months later. However, not everyone infected shows symptoms.

Common symptoms of Chlamydia include:

  • pain when peeing
  • unusual discharge from the vagina, penis, or anus
  • women may experience pain in the tummy, bleeding between periods or after sexual intercourse
  • men may experience pain and swelling of the testicles

In some people, symptoms may disappear after a few days. A person infected with Chlamydia can transmit the infection to others, even when they have no symptoms.

What you can do

A person with symptoms, or who suspects they have Chlamydia, is advised to contact their local sexual health service or GP to get advice and support on testing, diagnosis, and treatment.

Pregnant women who think they may have Chlamydia are advised to speak to their GP or midwife.


There are several steps that can be done to avoid catching and passing on Chlamydia to others:

  • practice safe sex by using condoms consistently and correctly
  • avoid sharing and wash sex toys after use
  • if at risk, have regular screening for Chlamydia and other STIs

Further information

Last updated 25 January 2024