What Is Chlamydia?


Chlamydia is the most common sexually transmitted infection globally.1 It is caused by a bacteria called Chlamydia trachomatis. Chlamydia is of huge public health importance, especially as most people don’t have symptoms and there is no vaccine available.2 The World Health Organisation (WHO) estimates that in 2020, 129 million people were infected worldwide. In the UK, particularly, the majority of diagnoses are amongst young people between 15-24 years.

Sexually transmitted infections (STIs) are primarily transmitted through unprotected sexual contact including vaginal, anal, and oral sex. Other means of transmission are mother-to-child during pregnancy, childbirth, and breastfeeding. According to the World Health Organization (WHO), globally, there are over 1 million new cases of STIs daily. STIs are caused by more than 30 different bacteria, viruses and parasites. Whilst bacterial and parasitic infections can be cured, viral infections can not. 

In the developed world, the most high-risk groups are sexually active adolescents and young people between the ages of 15-24 years. In the UK, this age group had 87,905 new cases of chlamydia in 2021. Other high-risk groups are people assigned male at birth (PAMAB) who have sex with other PAMAB, people who have multiple sexual partners (MSPs), vulnerable communities and young women.3,4,5

In England, the recommendation is for sexually active people assigned female at birth (PAFAB) under 25 years to have a chlamydia test once a year, and when they have sex with new or casual partners.  Sexually active PAMAB under 25 years are advised to have a chlamydia test once a year if they are not using condoms with new or casual partners. In addition, free and confidential chlamydia tests are available at sexual health clinics, genitourinary medicine (GUM) clinics and GP surgeries.

Causes of chlamydia

Chlamydia is a bacterial infection caused by Chlamydia trachomatis.6 The main mode of transmission is through unprotected sex (vaginal, anal, or oral sex) or contact with infected genital fluids (infected semen or vaginal fluid that enters the eye, sharing unwashed sex toys, skin-to-skin genital contact even if no penetration, orgasm, or ejaculation). Chlamydia can also be transmitted vertically (mother-to-child transmission). It is important to note that Chlamydia cannot be transmitted through casual contact, such as kissing, hugging, or sharing baths, towels, swimming pools, toilet seats or cutlery.

Signs and symptoms of chlamydia

Most people who have a chlamydia infection are asymptomatic. The type of symptoms, if developed, depends on the site of infection.1,2 

In people assigned female at birth (PAFAB), infection can cause:

  • Cervicitis: Infection of the cervix which can result in vaginal discharge, abdominal pain, dysuria (painful urination), postcoital bleeding and intermenstrual bleeding (bleeding between periods)
  • Pelvic inflammatory disease (PID): Infection of the reproductive system which includes the womb, fallopian tubes and ovaries. It commonly presents as abdominal or pelvic pain, with or without signs and symptoms of cervicitis. Other symptoms include nausea, vomiting, fevers, chills, low back pain, dyspareunia (painful intercourse), dysuria (painful urination), or postcoital bleeding (bleeding after intercourse)
  • Urethritis: Infection of the urethra which can cause urethral discharge and frequent urination
  • Perihepatisis: Chronic manifestation of pelvic inflammatory disease and can be associated with salpingitis (inflammation of the fallopian tubes) and right upper quadrant or pleuritic (lung) pain
  • Proctitis: Inflammation in the rectum 

In people assigned female at birth (PAFAB), infection can also cause urethritis and proctitis, as well as:

  • Epididymitis: Infection of the testicles which can cause testicular pain and tenderness, scrotal swelling and fever
  • Prostatitis: Infection of the prostate which can cause (painful urination), pelvic pain, and pain with ejaculation

If infection is passed on to a foetus in utero then this can cause:

  • Premature rupture of the membrane of the amniotic sac
  • Low birth weight and stillbirth
  • Conjunctivitis
  • Pneumonia: usually occurs between 4 to 12 weeks after birth, though almost all infants have symptoms before eight weeks

Management and treatment for chlamydia

The WHO STI guideline recommends treating chlamydia with antibiotics:

  • Oral Doxycycline: 100 mg twice a day for 7 days
  • Oral Azithromycin: single dose of 1g if there are no complications, or followed by 500mg on a day for 2 days7

 Second choice treatment options are:

Doxycycline and Ofloxacin are not recommended for use during pregnancy.8 A pregnancy test should be carried out to rule out pregnancy before starting these treatments.

In the UK, the NHS advises abstaining from any form of sex until doxycycline and azithromycin treatment is complete. It is also important that current and recent sexual partners are tested and treated to help stop the spread of the infection.


How is chlamydia diagnosed?

Diagnostic tests for chlamydia urogenital infections may be direct (detects the chlamydia bacteria itself) or indirect (detects antibodies produced by the body to defend itself against the bacteria).9

The gold standard of direct tests is known as nucleic acid amplification testing (NAAT) which detects the genetic material (DNA or RNA) of the bacteria.9 Samples used for NAAT are vaginal or endocervical swabs (PAFAB) and first-catch urine or urethral swab (PAMAB).  Because chlamydia infection can coexist with other STIs, it is recommended to test for other STDs.1 In the absence of tests, treatment is determined by the clinical presentation.

How can I prevent chlamydia?

To prevent chlamydia you should use a barrier method of contraception, such as a condom while having sex, particularly if you have a new sexual partner. You can stop the spread of chlamydia you can:

  • Use a condom every time you have vaginal or anal sex
  • Use a condom to cover the penis during oral sex
  • Use a dam (a thin, soft piece of plastic or latex) to cover female genitals during oral sex or when rubbing female genitals together
  • Not sharing sex toys

Who are at risk of chlamydia?

  • The most at-risk group are 15-24 years as they are most likely to engage in risky sexual behaviours involving unprotected sex
  • Those engaging in sex with a new sexual partner or multiple sexual partners (MSPs)
  • Gay men, bisexual men, and other men who have sex with men (GBMSM) are also at high risk3,4,5
  • Those of low socioeconomic status and in vulnerable communities are also a high-risk group. This relationship is not fully understood but could be linked to poor health-seeking behaviours

How common is chlamydia?

Chlamydia is the most commonly diagnosed STI in England. According to a UK Health Security Agency report, there were over 300 thousand new cases of STIs in England in 2021, with the most affected being young people aged 15 to 24 years (87,905 occurred in this group), certain Black ethnic groups, and gay, bisexual, and other men who have sex with men (GBMSM).

Can chlamydia go away on its own?

Chlamydia infection can exist for years without any symptoms developing. However, if left untreated, it can have profound effects on reproductive health. It is advised to see your GP if you suspect you have symptoms, are sexually active or are at risk. Chlamydia can be cured if treated.

What are the complications of chlamydia?

In PAFAB, pelvic inflammatory disease (PID), an infection of the reproductive system, is considered a complication of chlamydia that can lead to tubal infertility and ectopic pregnancy

If pregnant, chlamydia could be passed onto the baby and cause conjunctivitis and pneumonia, as well as premature birth and low birth weight. 

In PAMAB, epididymitis, an infection of the testicles, can cause pain and swelling of the testicles and can affect fertility. Chlamydia is also the most common cause of sexually acquired reactive arthriritis (SARA) where joints, eyes or urethra become inflammed. This can also affect PAFAB but it is more common in PAMAB.

When should I see a doctor?

You should consider seeing your doctor or getting tested at a sexual health clinic, genitourinary medicine (GUM) clinic or your GP surgery if:

  • you or your partner develop any symptoms of chlamydia
  • you had unprotected sex with a new partner or have multiple sexual partners
  • you or your partner have had unprotected sex with other people
  • you or your partner suspects having a sexually transmitted infection (STI)
  • you are pregnant or planning a pregnancy
  • you are offered a chlamydia test as part of the national chlamydia screening programme


Chlamydia is the most common sexually transmitted disease worldwide. It is caused by a bacteria called chlamydia trachomatis. High-risk groups are those who are 15-24, have unprotected sex and/or have multiple sexual partners. Most people with the disease are asymptomatic. Regular tests are recommended for early diagnosis as untreated chlamydia infection can result in complications that affect sexual and reproductive health. Chlamydia infection is a curable disease.


  1. Mohseni M, Sung S, Takov V. Chlamydia. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 [cited 2023 Mar 3]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK537286/
  2. Bébéar C, de Barbeyrac B. Genital Chlamydia trachomatis infections. Clin Microbiol Infect. 2009 Jan;15(1):4–10. Available from: https://pubmed.ncbi.nlm.nih.gov/19220334/
  3. Navarro C, Jolly A, Nair R, Chen Y. Risk factors for genital chlamydial infection: a review. Canadian Journal of Infectious Diseases [Internet]. 2002 [cited 2023 Mar 4];13(3):195–207. Available from: http://www.hindawi.com/journals/cjidmm/2002/954837/abs/
  4. Bavastrelli M, Midulla M, Rossi D, Salzano M, Calzolari E, Midulla C, et al. Sexually active adolescents and young adults: a high-risk group for chlamydia trachomatis infection. J Travel Medicine [Internet]. 1998 Jun [cited 2023 Mar 4];5(2):57–60. Available from: https://academic.oup.com/jtm/article-lookup/doi/10.1111/j.1708-8305.1998.tb00464.x
  5. Woodhall SC, Soldan K, Sonnenberg P, Mercer CH, Clifton S, Saunders P, et al. Is chlamydia screening and testing in Britain reaching young adults at risk of infection? Findings from the third National Survey of Sexual Attitudes and Lifestyles (Natsal-3). Sex Transm Infect [Internet]. 2016 May 1 [cited 2023 Mar 4];92(3):218–27. Available from: https://sti.bmj.com/content/92/3/218
  6. Rodrigues R, Sousa C, Vale N. Chlamydia trachomatis as a current health problem: challenges and opportunities. Diagnostics [Internet]. 2022 Aug [cited 2023 Mar 4];12(8):1795. Available from: https://www.mdpi.com/2075-4418/12/8/1795
  7. Recommendations for treatment of chlamydial infections [Internet]. World Health Organization; 2016 [cited 2023 Mar 6]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK379708/
  8. O’Connell CM, Ferone ME. Chlamydia trachomatis genital infections. MIC [Internet]. 2016 Sep 5 [cited 2023 Mar 6];3(9):390–403. Available from: http://microbialcell.com/researcharticles/chlamydia-trichomatis-genital-infections
  9. Meyer T. Diagnostic procedures to detect chlamydia trachomatis infections. Microorganisms [Internet]. 2016 Aug 5 [cited 2023 Mar 6];4(3):25. Available from: http://www.mdpi.com/2076-2607/4/3/25 
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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