In this article, find out everything you need to know about four of the most common Sexually Transmitted Infections, including their symptoms, causes and how to treat them.

What is an STI?

A sexually transmitted infection – or ‘STI’ for short – is an infection that is passed from one person to another through sexual contact.

What is the difference between an STI and an STD?

Although often used interchangeably, STI and STD have different meanings; the most noticeable of which is that while STI stands for sexually transmitted infection, STD stands for sexually transmitted disease.

Sexually transmitted infections are defined as invasions of the body by organisms such as parasites, viruses or bacteria. An STI on its own may not lead to any specific symptoms or cause any health problems, as someone might just be a carrier of the infection and may potentially be contagious.

Sexually transmitted diseases, on the other hand, cause specific health problems. A sexually transmitted disease will have originally begun as a sexually transmitted infection.

How are STIs diagnosed?

STIs are usually diagnosed by healthcare providers in one of the following ways:

  • Physical examinations
  • Blood tests
  • Swab tests

Common types of STIs

Discover four of the most common STIs and find out more about their symptoms, causes and treatments.


Chlamydia is one of the most common STIs in the UK[1], and it is particularly common in teenagers and young adults that are sexually active.

This STI most commonly affects the genitals, but it is possible to get chlamydia in the throat, eye or rectum.

Chlamydia affects men and women differently:

The symptoms of chlamydia in men

The most common symptoms of chlamydia in men include:

  • Discharge from the tip of the penis
  • Pain in the testicles
  • Itching and burning of the urethra (the tube that passes urine out of the body)
  • Pain when urinating

If left untreated, it can cause swelling in the testicles and the epididymis (the tubes that pass sperm from the testicles).

The symptoms of chlamydia in women

While at least 70%[1] of women with chlamydia reportedly don’t notice symptoms, the most common symptoms can include:

  • Abdominal pain or pain in the pelvis
  • Bleeding after sex or between periods
  • Painful sex
  • Unusual vaginal discharge
  • Pain when urinating

If left untreated, it can spread to the womb and lead to a serious condition called Pelvic Inflammatory Disease (PID), which can cause infertility and ectopic pregnancy.

Find out more about the complications of Chlamydia in our blog: 4 key facts about Chlamydia & how to buy Chlamydia treatment

What causes chlamydia?

Chlamydia is a bacterial infection that is contracted from unprotected vaginal, anal, and oral sex. It’s spread through infected bodily fluids (such as vaginal discharge and semen) and by sharing sex toys.

Chlamydia treatment

Chlamydia is a bacterial infection, so it is treated with a course of antibiotics. The most common antibiotics used to treat chlamydia are:

Discover medication for chlamydia treatment >

Genital Herpes

Genital herpes are painful blisters that form on or around the genitals, as well as the rectum and the thighs. Genital herpes cannot be cured, but it can be managed through medication.

The symptoms of genital herpes

As well as blisters, other symptoms of genital herpes can include:

  • Flu-like symptoms, including fatigue and fever
  • Pain when going to the toilet
  • Red areas or cracked skin
  • Burning or itching on and around the genitals

Women may also experience unusual vaginal discharge.

What causes genital herpes?

Caused by the Herpes Simplex Virus, genital herpes can be passed through unprotected vaginal, anal and oral sex – even if the person isn’t showing or experiencing any symptoms.

Caused by the Herpes Simplex Virus, once you have contracted the virus, it lays dormant until an outbreak occurs.

Genital herpes treatment

Aciclovir is an antiviral medication that is used to treat the condition. It limits how much the virus duplicates in the body, which helps to prevent outbreaks from occurring and limits how long the outbreaks last. Read Treat herpes fast with Aciclovir tablets to find out more.

Discover medication for genital herpes treatment >


Cystitis is a Unitary Tract Infection (UTI) that affects the bladder. This infection is very common for women and often gets better by itself, but in some cases, you will need antibiotics.

The symptoms of cystitis

The most common symptoms of cystitis include:

  • Pain or burning sensation when urinating
  • Pain in the lower abdomen
  • Cloudy and dark urine
  • Having the urge to urinate but not being able to

What causes cystitis?

Cystitis is caused by bacteria entering the urinary tract and travelling up to the bladder. There is no specific cause of cystitis, but there are things that can increase the likelihood of contracting it, such as:

  • Having sex
  • Using condoms or a diaphragm that are coated in spermicides (chemicals that destroy sperm)

To help prevent cystitis, it is advisable to urinate as soon as possible after having sex.

Cystitis treatment

Cystitis often clears up by itself, but alkali-salt sachets can be used to relieve the symptoms. They work by neutralising the acid in the bladder that is created by the bacteria.

However, if symptoms persist, cystitis can be treated with a course of antibiotics.

Discover medication for cystitis treatment >

Cold Sores

Cold sores are caused by the Herpes Simplex Virus, they are very common and often clear up within 10 days. There is no cure for cold sores, but symptoms can be managed through medication.

The symptoms of cold sores

People tend to feel a burning or itching sensation on their face up to 48 hours before a cold sore forms, and then blisters full of fluid will appear. These blisters will burst and scab over, normally clearing up within 10 days. This process can be very uncomfortable and painful.

What causes cold sores?

Caused by the Herpes Simplex Virus, once you have contracted the virus, it lays dormant until an outbreak occurs. Cold sores are contagious from as soon as you start to feel the symptoms and up until they have completely healed.

You can contract the virus from close skin-to-skin contact with someone who is experiencing symptoms, such as sex or even kissing.

Cold sores treatment

Cold sores can be managed with either antiviral tablets or cream. Both forms of treatment should be used at the first signs of infection, but the cream can be started at the blister stage – helping to speed up the healing process.

Discover medication for cold sores treatment >

Myth busting: Bacterial Vaginosis (BV) is not an STI

Bacterial Vaginosis is an infection around the vagina. It is caused when the normal balance of bacteria is changed. Often, women don’t experience any symptoms, but symptoms can include:

  • Unusual discharge that has a strong odour
  • Soreness in and around the vagina

BV is not an STI, but it means you are more prone to contracting an STI, as the vagina is less acidic.

How to treat BV

Medications such as gels and pessaries are provided to restore the vagina to its natural PH level and relieve symptoms, but in some cases, you can be prescribed antibiotics.

Discover bacterial vaginosis treatment >

General prevention of STIs

There are some things you can do to lessen the likelihood of contracting an STI, such as:

  • Use barrier methods of contraception, such as condoms
  • Ensure you, and any new sexual partners, have regular STI tests
  • Have an honest discussion with any new sexual partners to find out about their previous sexual history

Although all of our content is written and reviewed by healthcare professionals, it should not be substituted for or used as medical advice. If you have any questions about your health, please speak to your doctor.

Authored Jul 28, 2022 by Joseph Issac, MPharm
Reviewed Feb 01, 2023 by Prabjeet Saundh, MPharm