What is an STI?

Reviewed by Dr Bhavini Shah

What are STIs? STI stands for 'sexually transmitted infection', which are passed between people through unprotected sex or genital contact. Recognising STI symptoms early is important to ensure you can get tested and treated as quickly as possible.

What you need to know

• STIs vs. STDs: As their names suggest, STIs are infections while STDs are the diseases caused by these infections. Find out more here.
• Symptoms and testing: STIs like HPV and chlamydia can be symptomless. If you’re sexually active, regular testing can be crucial for early detection and treatment.
• Prevention and treatment: Protect yourself with condoms and being aware of you and your partner’s sexual health history. Effective treatments are available for some STIs, so seek medical advice if needed.

How common are STIs?

According to current government data, new STI diagnoses decreased by 8.8% between 2023 and 2024. However, infectious syphilis diagnoses increased by 1.7% in the same time frame. Diagnoses rates of STIs are the greatest in:

• Young people aged 15 to 24 years
• Gay, bisexual and other men who have sex with men (GMBSM)
• Some minority ethnic groups

If you fall under any of the groups listed above, are sexually active and have not been tested, consider doing so.

What are the most common STIs?

The most commonly diagnosed STIs in England in 2024 were chlamydia, gonorrhoea, genital herpes and genital warts (which can be caused by HPV). We’ll take a look at each of these STIs below, highlighting symptoms, testing and showing where and how you can get treatment.

Chlamydia

Chlamydia is one of the most common STIs, most prevalent in young people aged 15-24. Left untreated, it can cause Pelvic Inflammatory Disease (PID). PID is a major cause of infertility and ectopic pregnancy (where an egg implants itself outside the womb), risking the lives of both mother and baby.

Chlamydia symptoms in women

• Pain or bleeding while having sex, or afterwards
• Bleeding outside periods - or heavier bleeding than usual during periods
• Soreness when you urinate
• Unusual vaginal discharge
• Pain in tummy or pelvic area

Chlamydia symptoms in men

• Pain in the testicles or when urinating
• Discharge from the penis which is usually white, cloudy or clear
• An itchy or burning feeling in the urethra (the tube in the penis that carries urine)

Chlamydia diagnosis and treatment

Getting tested is easy and uses a urine or swab test which you can get at your local sexual health clinic. These are also part of an online home testing kit. Treatment is with antibiotics.

At LloydsPharmacy Online Doctor, if you test positive using one of our kits and don’t have symptoms, our clinicians can provide you with free medical treatment.

"If you are sexually active and under 25 years old, the NHS recommends getting a chlamydia test every year – or every time you get a new partner.” - Dr Bhavini Shah

Gonorrhoea

It’s estimated that about 1 in 10 men and 5 in 10 women who have gonorrhoea don’t experience any symptoms. Some women experience mild symptoms that can be mistaken for something less serious, such as a bladder infection. It is passed on through unprotected oral, vaginal or anal sex, as well as sharing unclean sex toys.

If gonorrhoea is left untreated it can damage fertility, lead to pelvic inflammatory disease and infections of the brain, blood or heart, cause miscarriages and cause babies to get blood infections or be born blind.

Gonorrhoea symptoms in women

• Heavy bleeding during periods, bleeding outside periods, bleeding after sex
• A vaginal discharge, usually watery and thin or greeny-yellowy coloured
• Pain around the tummy or pain when urinating

Gonorrhoea symptoms in men

• A greeny-yellow discharge from the penis
• A burning feeling when urinating
• A swollen foreskin
• Pain or tenderness of the testicles

Gonorrhoea diagnosis and treatment

Testing is done via a swab or urine test at your local sexual health clinic, or you can order a home testing kit online.

Treatment is with an antibiotic injection and tablet at your local sexual health clinic.

Find out more about chlamydia vs gonorrhoea in our guide. 

HPV

HPV is the name for a group of common viruses that affect the skin or the moist membranes that line your body – such as the cervix, the anus and the mouth and throat. HPV can be carried by both men and women.

Certain strains of HPV can be caught via sexual intercourse or oral sex. Skin contact during sex can also cause it to spread, meaning using a condom does not always stop it.

HPV can cause genital warts or abnormal changes in the cells in the cervix that can sometimes turn into cancer. Most people who have it do not realise and do not have any problems. 

Women aged 25 to 64 in England are offered cervical screening to look out for signs of cancer. It's recommended to have one every three years, this changes to every five years once over 50.

HPV vaccines were first licensed in 2006. The vaccine is offered to boys and girls aged 12-13 as part of the National Immunisation Programme. It is also offered on the NHS for women who didn’t receive the vaccination at school and are under 25, as well as men under 45 who have sex with men.

Genital herpes

This highly contagious STI affects both men and women and can be caught during sex even if a condom is used, via infected skin. It can also be caught when a cold sore or an infected finger touches your genitals.

Genital herpes symptoms in men / women

• Vaginal discharge
• Painful blisters on or around the genitals
• Soreness when urinating
• Burning, tingling or itching around the genitals

Genital herpes diagnosis and treatment

• There is no cure, but antiviral medicines can be taken to relieve the symptoms
• Cream can also be applied to help with pain and ‘flare ups’
• Medication can be taken daily to help prevent flare-ups. This is usually advised if there are six or more flare-ups a year

If you do experience six or more flare-ups of genital herpes each year, you might consider our herpes treatment service. Take a short, free online consultation today to see if you’re suitable.

STI symptoms

Not all STIs cause noticeable symptoms but you should certainly be aware of the common ones. Several STIs, like chlamydia, can be asymptomatic - i.e. they display no symptoms but can cause long-term complications like infertility. 

The World Health Organization (WHO) estimates that more than 1 million STIs are caught every day by people worldwide aged 15-49, the majority of them being asymptomatic.

If you display any of the symptoms listed below, get a check-up as soon as possible. Our online sexual health clinic offers reliable and discreet STI tests, treatment and more.

STI symptoms in all genders

• Pain when urinating
• Itching, burning or tingling around the genitals
• Blisters, sores, lumps or a rash around the genitals or anus

STI symptoms in those assigned female at birth

• Yellow or green vaginal discharge
• smelly discharge (often fish-like)
• Bleeding between periods
• Bleeding after sex
• Pain during sex
• Stomach pain

STI symptoms in those assigned male at birth

• Discharge from the penis tip
• Irritation of the urethra
• Swollen testicles

“Even if you feel in perfect health after unprotected sex and show no symptoms, that doesn’t mean you’re safe from an STI. If you recently put yourself at risk of catching an STI (e.g. through unprotected sex with a new partner) you should get tested. Our STI tests are intended for patients that are not currently showing symptoms.” – Dr Bhavini Shah

If you have severe symptoms such as pain, abdominal pain or swollen testicles then you should see a doctor without delay.

How do you test for STIs?

If you think you might have an STI you must get tested. (If you have severe symptoms then see a doctor without delay.) Visit your GP or local GUM clinic to pick up an STI test. 

Alternatively, you can order a test online from our online doctor service. The test is delivered to your home, then posted to our laboratory without the need for a face-to-face appointment. Visit our online STI test clinic for more information.

Prevention and safe sex practices

While there are a number of treatment options available for STIs, the most effective way to look after your sexual health is to practice safe sex. Prevention takes less time than treatment. Make sure you:

• Use condoms for oral, vaginal and anal sex (making sure they're in date and intact)
• Ask any new sexual partners about their sexual history - and if they've recently taken an STI test
• Be aware of your own sexual health and history
• Test regularly if you're engaging in casual sex with different partners
• Be aware of any unusual smells, discharge or growths
• Remember you can always say no if you feel at risk or uncomfortable

STI treatments

STIs such as chlamydia and gonorrhoea should clear up quickly once treated, although reinfection can occur. Each STI requires a different treatment. For more information visit our STI treatment page.