Sexually Transmitted Infections (STIs)
Symptoms, Causes & Treatment
Everything you need to know about sexually transmitted infections: how they spread, what symptoms to look for, how they are diagnosed, and the treatments available.
What Are STIs?
Sexually transmitted infections (STIs) are infections passed from one person to another through sexual contact -- vaginal, anal, or oral sex, and sometimes through close genital skin-to-skin contact. STIs are extremely common. In the UK, over 390,000 new STI diagnoses are made each year, and the true number of infections is significantly higher because many STIs cause no symptoms and go undiagnosed.
STIs are caused by bacteria, viruses, or parasites. Some are easily curable with antibiotics (chlamydia, gonorrhoea, syphilis, trichomoniasis). Others are caused by viruses and cannot be cured but can be effectively managed with treatment (genital herpes, HPV, hepatitis B). Left untreated, STIs can lead to serious complications including pelvic inflammatory disease (PID), infertility, ectopic pregnancy, chronic pain, and -- in the case of syphilis -- life-threatening illness.
STIs affect people of all ages, genders, and sexual orientations. They are not a reflection of personal hygiene, character, or lifestyle choices. The most effective tools for sexual health are regular testing, prompt treatment, open communication, and consistent condom use
What Are the Symptoms of STIs?
STI symptoms vary widely by infection. Many STIs cause no symptoms at all. The following are the most common presentations to be aware of.
Unusual discharge (penis, vagina, rectum) · Sores, blisters, or ulcers on genitals or mouth · Burning or pain when urinating · Rash on palms, soles, or trunk · Swollen lymph nodes in groin · Itching or irritation around genitals · Pelvic or testicular pain · Warts or growths on genitals
Severe pelvic pain with fever (possible PID) · High fever with rash and joint pain (disseminated gonorrhoea) · Neurological symptoms (confusion, headache, stiff neck) with STI history — possible syphilitic meningitis · Inability to urinate due to urethral swelling
Seek emergency care if STI symptoms are accompanied by high fever and severe pelvic or abdominal pain (possible PID or systemic infection), neurological symptoms, or inability to urinate.
What Causes STIs?
STIs are caused by bacteria, viruses, or parasites transmitted through sexual contact. Understanding how each infection spreads is the foundation of prevention and partner management.
STIs are transmitted through unprotected vaginal, anal, or oral sex. Some (herpes, HPV, syphilis) can spread through skin-to-skin contact even without penetration or visible symptoms.
Many STIs, particularly chlamydia, gonorrhoea, and early HIV, cause no symptoms at all. Regular testing is the only way to detect them — waiting for symptoms means delayed treatment and onward transmission.
Untreated STIs can cause PID, infertility, ectopic pregnancy, epididymo-orchitis, and increased HIV transmission risk. Syphilis causes serious cardiovascular and neurological damage if untreated for years.
Chlamydia trachomatis infects columnar epithelial cells. It replicates intracellularly, causing a subclinical inflammatory response that silently damages the fallopian tubes and epididymis over time.
Condoms reduce STI transmission by 85–98% when used consistently and correctly. They are less effective for infections spread by skin contact (herpes, HPV, syphilis) where lesions extend beyond the condom-covered area.
STI co-infection is common. Gonorrhoea and chlamydia frequently co-exist (15–40% co-infection rate). HIV significantly increases susceptibility to all other STIs and their complications.
Key Risk Factors
How Are STIs Diagnosed?
A full STI screen includes urine, swabs (genital, rectal, pharyngeal), and blood tests. The specific tests depend on your sexual practices and symptoms. Results are typically available within 1–2 weeks. Self-sampling kits are available online from NHS services.
How Are STIs Treated?
Treatment depends on the specific STI. Bacterial infections are cured with antibiotics; viral infections are managed with antivirals or antiretrovirals. All treatments require partner notification.
Supportive Measures
Complete the full antibiotic course even if symptoms resolve early. Abstain from sex until you and all notified partners have completed treatment. Attend test of cure for gonorrhoea. Inform recent sexual partners — clinics can do this anonymously on your behalf.
Recurrent or Persistent STIs
Some STIs recur or persist: herpes reactivates lifelong; HPV may persist for years; gonorrhoea can be reinfected immediately after treatment; chlamydia risk persists with new partners. Developing a regular testing habit and consistent condom use with new partners is the foundation of long-term sexual health.
When Should You Get Tested?
Severe pelvic pain with fever (possible PID) · Testicular pain with fever (possible epididymo-orchitis) · Neurological symptoms with known STI history · Suspected meningitis in a person with secondary syphilis — call 999.
You have had unprotected sex · A partner has been diagnosed with an STI · You have symptoms of an STI · You have not been tested in the past year and are sexually active · You want to start PrEP or discuss HIV prevention.
How to Prevent STIs
Most STIs are preventable. Consistent testing, condom use, and vaccination are the cornerstones of sexual health protection.
Annual STI testing is recommended for all sexually active people. Those with multiple partners should test every 3 months. Free NHS testing is available at sexual health clinics, online via SHGM/SH:24, and from GP surgeries.
Use condoms consistently for vaginal, anal, and oral sex with new or casual partners. Dental dams reduce transmission during oral-vulval contact. Lube reduces condom breakage and anal mucosal trauma.
Notify all recent sexual partners when diagnosed with an STI so they can be tested and treated. Partner notification can be done directly or anonymously through the sexual health clinic (contact tracing service).
HPV vaccination prevents 90% of cervical cancer and most genital warts. Hepatitis B vaccine is recommended for all MSM, PWID, and sexual health clinic attendees not previously vaccinated.
PrEP (Truvada or Descovy) is available free on the NHS at sexual health clinics for eligible high-risk individuals. It is over 99% effective when taken as prescribed. Regular HIV and kidney function testing is required during use.
If you are HIV-positive and on effective ART with an undetectable viral load, you cannot transmit HIV to sexual partners (U=U — Undetectable = Untransmittable). This is one of the most important advances in HIV medicine.
Speak to a Clinician About STIs
Through The GP Service, you can access confidential STI assessment, testing referrals, and antibiotic prescriptions for treatable infections — discreetly and without a waiting room.



Expert clinical advice, when you need it.
STI FAQs
STI testing is recommended annually for all sexually active people. Those with new or multiple partners should test every 3 months. Testing is free at NHS sexual health clinics and from online services (SH:24, SHGM). You do not need symptoms to be tested — most STIs cause no symptoms. A full STI screen covers chlamydia, gonorrhoea, syphilis, HIV, hepatitis B, and hepatitis C. Specific sites tested (urine, swabs, blood) depend on your sexual practices. Results are usually available within 1–2 weeks. Positive results trigger automatic partner notification advice.
Not all STIs are curable, but all are treatable. Bacterial STIs — chlamydia, gonorrhoea, and syphilis — are fully curable with antibiotics when diagnosed promptly. Viral STIs — HIV, herpes, HPV, and hepatitis B — cannot be cured but can be effectively managed. HIV is controlled with ART to the point of undetectability. Herpes is managed with antivirals that suppress outbreaks. HPV often clears naturally within 1–2 years in people with healthy immune systems. Genital warts can be treated and removed, though the underlying HPV virus may persist. The key message: all STIs are manageable with prompt diagnosis and treatment.
Without treatment, chlamydia can persist silently for months or years, causing progressive damage to the reproductive tract. In women, untreated chlamydia causes pelvic inflammatory disease (PID) in 10–15% of cases, leading to tubal factor infertility, chronic pelvic pain, and ectopic pregnancy risk increased 6–10-fold. In men, chlamydia causes epididymo-orchitis, which can impair sperm production and quality. Reactive arthritis (formerly Reiter's syndrome) — involving joint inflammation, eye inflammation, and urethritis — is a rare but recognised complication. These complications are entirely preventable with prompt diagnosis and a simple 7-day course of doxycycline.
Gonorrhoea is increasingly difficult to treat due to rising antibiotic resistance. The current UK first-line treatment is a single intramuscular injection of ceftriaxone 1g. A test of cure is required 2 weeks later to confirm eradication. Azithromycin is no longer recommended routinely due to resistance. Multidrug-resistant gonorrhoea has been identified in multiple countries, including the UK. If you have been treated for gonorrhoea and symptoms persist, return to the clinic immediately — do not take additional antibiotics without guidance. Treatment of all recent sexual partners is essential to prevent reinfection.
HPV vaccination is highly effective before first sexual contact but still provides meaningful protection after sexual debut. The NHS offers Gardasil 9 to all children aged 12–13 regardless of gender. Catch-up vaccination is available free up to age 25 for women and men who have not been vaccinated. MSM can receive free vaccination at sexual health clinics up to age 45. For people already exposed to HPV, the vaccine still protects against strains they have not yet encountered. HPV vaccination combined with regular cervical screening has the potential to virtually eliminate cervical cancer in the UK.
Yes. HIV is now a manageable chronic condition, not a death sentence. With antiretroviral therapy (ART), people with HIV can live long, healthy lives with a normal life expectancy. ART suppresses the virus to undetectable levels in the blood, meaning it cannot be transmitted sexually (Undetectable = Untransmittable, or U=U). Most people on modern ART take a single tablet daily and experience minimal side effects. HIV does not prevent you from having children, working, or having a fulfilling life. In the UK, HIV care is excellent and free on the NHS. The biggest barrier to good outcomes is late diagnosis — testing early is life-saving.
PEP (post-exposure prophylaxis) is a 28-day course of antiretroviral drugs that prevents HIV infection after a potential exposure. It must be started within 72 hours — the sooner the better. PEP is available from A&E departments, sexual health clinics, and some GUM clinics 24/7. It is free on the NHS for NHS patients. PEP is not a substitute for PrEP or condoms and should be used only in genuine emergencies (unprotected sex with a known HIV-positive person not on suppressive ART, condom failure, sexual assault, or needlestick). After completing PEP, an HIV test at 45 days and 3 months confirms whether infection was prevented.
PE affects approximately 20–30% of men at some point — making it the most common male sexual dysfunction. Many men suffer in silence due to embarrassment, assuming it is shameful or untreatable. In reality, PE is a well-recognised medical condition with highly effective treatments. Dapoxetine (a short-acting SSRI licensed specifically for PE) can more than triple ejaculatory latency time in clinical trials. Behavioural techniques (stop-start, squeeze) are effective for lifelong PE. Combining medication with psychological or couples therapy produces the best outcomes. Most men with PE who seek treatment experience significant improvement.
