Chlamydia: Symptoms, Causes, Complications & Treatment UK
Key fact: Chlamydia is the most commonly diagnosed sexually transmitted infection (STI) in the UK. Up to 70% of women and 50% of men with chlamydia experience no symptoms at all — meaning most people carry the infection without knowing it. Testing is the only way to know for certain.
230k+
New chlamydia diagnoses in England each year (UKHSA 2023)
70%
Of women with chlamydia who have no symptoms
15–24
Age group accounting for the majority of diagnoses in the UK
What Is Chlamydia?
Chlamydia is a bacterial infection caused by Chlamydia trachomatis, an obligate intracellular pathogen — meaning it can only survive and reproduce inside human cells. It is the most common bacterial STI in the UK and one of the most common infections in young people globally.
The bacterium primarily infects the epithelial cells lining the urogenital tract: the cervix and urethra in women, and the urethra in men. It can also infect the rectum (via anal sex), the throat (via oral sex), and the eyes (causing conjunctivitis, most clinically significant in newborns). Because C. trachomatis replicates silently inside cells, it typically provokes little immediate immune response — which is why symptoms are absent in most cases and the infection can persist for months or years without detection.
The UK Health Security Agency (UKHSA) reported over 230,000 new chlamydia diagnoses in England in 2023, with the highest rates consistently found in people aged 15–24. The figure represents diagnosed cases only; actual prevalence is considerably higher given the asymptomatic nature of the infection.
How Is Chlamydia Spread?
Chlamydia is transmitted through unprotected sexual contact with an infected person. It is not spread by casual contact such as touching, kissing, sharing towels, toilet seats, or swimming pools.
Transmission routes include:
- Vaginal sex — the most common route of transmission
- Anal sex — can result in rectal chlamydia in the receptive partner
- Oral sex — less efficient route; can cause pharyngeal (throat) chlamydia
- Sharing sex toys without thorough cleaning or changing condoms between partners
- Mother to baby during childbirth (vertical transmission), which can cause neonatal conjunctivitis and pneumonia
Condoms, when used consistently and correctly, substantially reduce transmission risk, though they do not eliminate it entirely since chlamydia can be present on surfaces not covered by a condom. A sexual partnership with an infected person carries an estimated 40–70% probability of transmission over the course of the relationship.
No symptoms does not mean no infection. A sexual partner can transmit chlamydia without knowing they are infected. You cannot tell whether someone has chlamydia from their appearance, history, or absence of symptoms. Testing is the only reliable method of detection.
Chlamydia Symptoms in Women
Around 70–80% of women with chlamydia experience no symptoms, which is why testing is essential. When symptoms do occur, they include unusual vaginal discharge, pelvic pain, dysuria, and bleeding between periods or after sex. The primary infection site is the cervix (cervicitis).
Chlamydia Symptoms in Men
Men are more likely than women to experience symptoms, but still approximately 50% have none. The primary site of infection in men is the urethra (urethritis). Symptoms, when present, typically appear 1–3 weeks after exposure.
- Urethral discharge — a watery or cloudy discharge from the tip of the penis, often present in the morning
- Dysuria — burning or stinging when urinating
- Testicular pain or swelling — may indicate spread to the epididymis (epididymo-orchitis), which requires prompt treatment
- Rectal discomfort or discharge — in men who have receptive anal sex and have acquired rectal chlamydia
Testicular swelling or pain: Unilateral testicular pain, tenderness, or swelling may indicate epididymo-orchitis — a complication of untreated urethral chlamydia. This requires urgent medical assessment and a full antibiotic course, not just standard single-dose chlamydia treatment.
Why Most People Have No Symptoms
Chlamydia trachomatis has evolved highly effective mechanisms to evade the host immune response. It replicates within membrane-bound vacuoles (inclusions) inside epithelial cells, effectively hiding from the immune system. This intracellular life cycle allows the bacterium to persist for prolonged periods without triggering the inflammatory cascade that produces noticeable symptoms.
The consequence is that chlamydia can be present in the body for months or years without symptoms, all the while causing subclinical (silent) inflammation. This chronic low-grade inflammation is the mechanism by which untreated chlamydia causes its most serious long-term complications, particularly in the female reproductive tract.
Complications If Chlamydia Is Left Untreated
Untreated chlamydia is one of the most common preventable causes of infertility and reproductive damage in the UK. In women it can lead to pelvic inflammatory disease (PID), tubal factor infertility, and ectopic pregnancy. In men, epididymo-orchitis is the primary risk.
Chlamydia in Pregnancy
Chlamydia during pregnancy requires prompt treatment as it can be transmitted to the newborn during delivery, causing neonatal conjunctivitis and pneumonia. Doxycycline is contraindicated in pregnancy — safe antibiotic alternatives are used instead.
Getting Tested for Chlamydia
The only reliable way to detect chlamydia is laboratory NAAT testing — symptoms alone cannot confirm or exclude the infection. Tests are available via home kit or sexual health clinic. The recommended window period is at least 2 weeks after potential exposure.
Chlamydia Treatment: An Overview
Chlamydia is curable with a short course of antibiotics. The current BASHH 2025 recommended treatments for uncomplicated urogenital chlamydia are:
| Treatment | Regimen | BASHH 2025 status |
|---|---|---|
| Doxycycline | 100 mg twice daily for 7 days | First-line (urogenital & rectal) |
| Azithromycin | 1 g as a single oral dose | Second-line / when doxycycline contraindicated |
| Erythromycin | 500 mg four times daily for 14 days | Alternative in pregnancy or macrolide/tetracycline intolerance |
All partners from the previous 6 months should be notified, tested, and treated regardless of their own test result. Abstain from sex for 7 days after completing treatment. Retest at 3 months to detect reinfection.
First-line treatment (BASHH 2025)
Doxycycline 100 mg
Twice daily for 7 days. Current first-line per BASHH 2025. Prescription following clinical assessment.
View doxycycline →Second-line treatment
Azithromycin 1 g
Single oral dose. Second-line or when doxycycline is contraindicated. Prescription following clinical assessment.
View azithromycin →Frequently Asked Questions about Chlamydia
How do you know if you have chlamydia?
You cannot tell from symptoms alone — most people with chlamydia have none. The only way to know is to get tested. A NAAT test from a urine sample (men) or vaginal swab (women) will accurately detect or exclude the infection. If you have had unprotected sex with a new partner, it is worth testing even if you feel completely well.
How long can chlamydia go undetected?
Chlamydia can persist for many months or years without causing symptoms. Cases have been documented where infection has remained undetected for over a year, causing progressive subclinical reproductive tract damage throughout that time. This is why routine testing — rather than symptom-driven testing — is so important in sexually active young people.
Can chlamydia go away on its own?
Although some evidence suggests that chlamydia can spontaneously resolve in a small proportion of cases without treatment, this is not reliable. Untreated chlamydia may persist for months to years and can cause serious complications — particularly PID and infertility in women — even without noticeable symptoms. Treatment with antibiotics is always recommended upon diagnosis.
Can you get chlamydia more than once?
Yes. Chlamydia infection does not produce protective long-term immunity. Having had chlamydia before offers no protection against future infection. Reinfection is common — particularly from an untreated partner — which is why retesting at 3 months after treatment is recommended, and why partner notification is essential.
Does chlamydia affect fertility?
Untreated chlamydia is one of the leading preventable causes of tubal factor infertility in women. Repeated or silent PID causes scarring of the fallopian tubes, which can prevent egg fertilisation and increase the risk of ectopic pregnancy. In men, untreated epididymitis can impair sperm transport. Early detection and treatment significantly reduces the risk of fertility complications.
How long after exposure should I wait before testing?
NAAT tests for chlamydia are accurate from approximately 2 weeks after exposure. Testing sooner risks a false-negative result if the infection is still in its early replication phase. If you test negative at 2 weeks and had significant exposure, BASHH recommends a repeat test at 6 weeks to confirm the result.
Can chlamydia be caught from oral sex?
Yes, though pharyngeal (throat) chlamydia is considerably less common than genital infection. It is usually asymptomatic and is transmitted by giving oral sex to an infected person. Pharyngeal chlamydia can be tested using a throat swab and requires antibiotic treatment; cure rates with standard regimens may be slightly lower than for urogenital infection, and test of cure is recommended.
Is chlamydia the same as gonorrhoea?
No. Chlamydia and gonorrhoea are distinct bacterial infections caused by different organisms (Chlamydia trachomatis and Neisseria gonorrhoeae, respectively). Both are sexually transmitted and can cause urethral, cervical, rectal, and pharyngeal infections, but they require different treatments. Co-infection is not uncommon, and testing for both is standard in sexual health clinics.
References
- UKHSA. Sexually transmitted infections and gonorrhoea: annual data tables. UK Health Security Agency, 2024. Available at: gov.uk/government/statistics/sexually-transmitted-infections
- BASHH. UK National Guideline for the Management of Infection with Chlamydia trachomatis. 2025 update. Available at: www.bashh.org/guidelines
- NICE. Chlamydia — uncomplicated genital: Clinical Knowledge Summary. 2025. Available at: cks.nice.org.uk
- Oakeshott P, Kerry S, Aghaizu A, et al. Randomised controlled trial of screening for Chlamydia trachomatis to prevent pelvic inflammatory disease: the POPI (prevention of pelvic infection) trial. BMJ. 2010;340:c1642.
- Haggerty CL, Gottlieb SL, Taylor BD, et al. Risk of sequelae after Chlamydia trachomatis genital infection in women. J Infect Dis. 2010;201(Suppl 2):S134–S155.
- NHS. Chlamydia. NHS.uk. 2023. Available at: www.nhs.uk/conditions/chlamydia/
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for advice about your individual circumstances. If you are experiencing symptoms or have been in contact with a diagnosed case of chlamydia, please seek assessment without delay. In a medical emergency, call 999.