Overview
Sexually transmitted infections (STIs) are infections passed from person to person primarily through sexual contact. Common STIs include chlamydia, gonorrhoea, syphilis, herpes, human papillomavirus (HPV), HIV, and trichomoniasis. Many STIs cause no symptoms. Early testing and treatment prevent complications and onward transmission.
How common is it?
STIs are extremely common. In England, around 400,000 STI diagnoses are made annually at sexual health clinics. Chlamydia is the most frequently diagnosed, particularly in people under 25. HPV affects most sexually active people at some point.
Causes and risk factors
Each STI has a specific pathogen. They are transmitted through vaginal, anal, or oral sex, and some through blood or skin-to-skin contact with infected areas.
Common risk factors
- Unprotected vaginal, anal, or oral sex
- Multiple sexual partners
- Young age (under 25 years)
- Not using condoms consistently
- Previous STIs (increases susceptibility)
- Sex work or partner who has multiple partners
- HIV infection (increases susceptibility to other STIs and vice versa)
- Some STIs transmissible perinatally or via blood (HIV, syphilis, hepatitis B)
Symptoms
- Many STIs are asymptomatic
- Unusual discharge from the penis, vagina, or rectum
- Burning or pain when urinating
- Sores, ulcers, or blisters on genitals or mouth (herpes, syphilis)
- Rash (syphilis secondary stage)
- Warts on or around the genitals or anus (HPV)
- Lower abdominal pain in women (pelvic inflammatory disease from chlamydia or gonorrhoea)
- Testicular pain and swelling
- Sore throat after oral sex
When to see a doctor
Get tested if you have had unprotected sex with a new or multiple partners, have symptoms, or a partner informs you they have an STI. Regular STI testing (annually or after each new partner) is recommended for sexually active people.
Diagnosis
Testing varies by STI: urine or genital swab for chlamydia and gonorrhoea; blood test for HIV, syphilis, and hepatitis B and C; swab of sores for herpes; visual diagnosis or biopsy for HPV warts. Home testing kits are available for some STIs.
Treatments
Antibiotics
Chlamydia: doxycycline 100mg twice daily for 7 days (azithromycin single dose is now less preferred due to higher failure rates). Gonorrhoea: ceftriaxone 1g IM single dose. Syphilis: benzathine penicillin IM.
Antiviral medications
Herpes: aciclovir or valaciclovir reduces severity and duration of episodes. Suppressive daily therapy reduces recurrence and transmission risk. HIV: antiretroviral therapy (ART) — see HIV section. Hepatitis B: antiviral therapy if chronic.
Partner notification and treatment
All sexual contacts within the infectious period should be notified and offered testing and treatment. This is essential to prevent reinfection and break transmission chains. Sexual health clinics facilitate partner notification confidentially.
Self-care and lifestyle
- Use condoms consistently and correctly with all new or non-tested partners
- Get vaccinated: HPV vaccine (up to age 45, NHS offers to under 25s), hepatitis B vaccine (offered to all high-risk groups)
- Test regularly if sexually active and not in an exclusive relationship
- Discuss PrEP with a sexual health clinic if at high risk of HIV
Prevention
Consistent condom use, regular testing, HPV and hepatitis B vaccination, and PrEP for HIV prevention are the cornerstones. Limiting the number of sexual partners reduces exposure risk.