The Complete Guide to Sexually Transmitted Infections (STIs)

STIs versus STDs? How are they treated? And can they be cured? Learn more in our ultimate guide to sexually transmitted infections.

Updated: Wednesday 12 January 2022

Complete STI Guide

What is an STI?

STI stands for sexually transmitted infection, an infection that is passed from person to person through sexual contact. Some STIs can be cured with treatment, whilst some cannot; in this case, treatment helps to manage the symptoms and reduce the likelihood of transmission. You may have heard the term STD being used too, which stands for sexually transmitted disease. The difference is the same as with infection compared to disease, in that infections can develop into diseases but they don’t always. STI is a broader term that is more commonly being used interchangeably as it is seen as a more approachable phrase. STIs can be bacterial or viral, and can be treated with appropriate medication.

Types of STI and how to treat them

Genital Herpes

A common STI in the UK, an estimated 23% of the adult population have antibodies for genital herpes (HSV-2). [1]

Approximately 80% of those with genital herpes do not display symptoms, a factor contributing to its prevalence. If you do experience symptoms, they can appear as small blisters that leave sores around the genitals, rectum, buttocks, and thighs, vaginal discharge, or pain when urinating. During an outbreak, these symptoms can last for up to 20 days.

Genital herpes is transmitted when an affected area comes into contact with either small cracks in the skin, or through the soft, moist lining on the genitals, rectum, or mouth. HSV-2 is typically passed through unprotected sex, however if you have sores on areas of the body that contraception such as condoms doesn’t fully cover, the virus can still be transmitted. It’s important to remember that the virus might be active even when no symptoms show. It is also possible to get genital herpes symptoms via HSV-1 (oral herpes) when receiving oral sex from a person that has a cold sore.

You cannot cure HSV-2, and the virus lies dormant until reactivation. However, when the virus is active, antiviral medication can be taken to quickly clear the symptoms. Aciclovir is the most cost-effective option for treating a genital herpes outbreak, where a tablet is taken 3 times per day for 5 days. Valtrex, the branded version of Valaciclovir, comes in a slightly stronger 500 mg dose per tablet, but instead only 2 are taken per day. The medication is most effective when taken as soon as symptoms are noticed; a tingling or itching sensation can often be the first indication of an outbreak. If you get genital herpes outbreaks frequently, suppression packs are a good option to consider, involving taking a lower daily dose of Aciclovir or Valaciclovir for a 3 month period.

Oral Herpes (Cold Sores)

Although not usually referred to as an STI, over two-thirds of Brits have HSV-1, another strain of the herpes simplex virus. [2]

HSV-1 can cause sores on the lips or mouth (known as cold sores), gums, tongue, or inside the cheeks. Sometimes the virus can also cause muscle aches or fevers. The cold sores can appear in clusters, before breaking down and scabbing over.

The HSV-1 infection is highly contagious, contracted when you come into close contact with someone with the infection. This can occur via skin contact, which is why it’s common for many individuals to get infected as children. Coming into direct skin contact with herpes blisters or cold sores carries the most risk of becoming infected, particularly if kissing when the virus is active.

To treat oral herpes, antiviral medication can fight the infection. Treatment such as Aciclovir or Valtrex won’t provide a cure for HSV-1, but will reduce the severity and length of the outbreak. Bonjela cold sore cream is a treatment that stops an outbreak by blocking UV radiation, an environmental trigger that encourages the activation and multiplication of the virus. A different approach is with Compeed Cold Sore Patches, a discreet, gel-lined covering that speeds up the healing process and reduces scabbing.

Genital Warts

Genital warts are one of the most frequently occurring viral STIs in the UK, with continually rising incidence rates.[3]

The infection causes small growths that appear on the genitals or sometimes the anus. They can appear on their own or in clusters, and can be itchy or painless. The spots are usually found around the opening of the vulva, inside the vagina, on the penis or scrotum, in or around the anus, or on the upper thighs. However, those with the virus may not display any visible symptoms, or symptoms may appear many years after infection.

Genital warts are caused by HPV (human papillomavirus), typically the type 6 and type 11 strains. It’s passed on through skin-to-skin contact, even if genital warts are not visible. This is most common through penetrative sex, genital-to-genital contact, sharing sex toys, or oral sex in rare cases. Hugging, kissing or sharing towels cannot pass on HPV.

Treating genital warts can be done at home with prescription topical creams. Warticon cream is a powerful antiviral cream that suppresses HPV, preventing warts from developing and spreading. The treatment typically takes a matter of weeks to clear genital warts, one of the fastest treatments available. An alternative is Aldara cream, containing the immune response modifier imiquimod that stimulates the immune system to activate its natural defence mechanism. The treatment is applied and left on overnight, and has similar efficacy to Warticon in the short term. Aldara is thought to be more effective at preventing long term recurrences of genital warts, although it may take longer to clear a current outbreak than Warticon. Although usually harmless to your health, genital warts may grow larger if left untreated, which can be unpleasant and uncomfortable. For hard-feeling warts, your doctor might recommend having them removed via a heating or freezing treatment.


As the most common STI in the UK, chlamydia accounted for 49% of all STI cases in 2019.

Between 70% and 90% of affected individuals are asymptomatic, with no obvious signs or symptoms appearing. [4] You may experience symptoms a few weeks after becoming infected, which can include pain when peeing, bleeding during or after sex, lower abdominal pain, swelling of the testicles or unusual discharge from the penis, vagina or rectum.

Chlamydia trachomatis is the bacteria responsible for causing chlamydia and is present in the semen or vaginal fluid of those who are infected. It is therefore passed during unprotected sex (including oral sex), genital-to-genital contact and sharing unwashed sex toys. Chlamydia cannot be passed on through the saliva, such as kissing or sharing cutlery, or through close contact like hugging.

A course of antibiotics are used to treat chlamydia, and the two most commonly prescribed treatments are Azithromycin or Doxycycline. You should not have sex for 7 days after the treatment, and symptoms should quickly start to improve within a few days. Antibiotics are successful at clearing the infection in 95% of individuals, but you should get tested 3 months after finishing your treatment to check for reinfection, especially if you’re in the high-risk under-25s age group. [5]


As the second most common bacterial STI in the UK behind chlamydia, gonorrhoea can lead to infertility if left untreated. [6]

Signs of gonorrhoea include green or yellow discharge from the vagina or tip of the penis, a burning sensation when peeing, pain in the lower abdomen or painful swelling of the foreskin. The infection can also infect the rectum, eyes and throat, causing conjunctivitis in the eyes or pain in the rectum. With that said, half of women and a tenth of men won’t have any clear symptoms, or may experience symptoms many months after becoming infected.

Neisseria gonorrhoeae, or gonococcus, is the bacteria that causes gonorrhoea and is transmitted through semen or vaginal fluid. This can occur through unprotected penetrative or oral sex, as well as sharing sex toys and not washing thoroughly in between uses. It is not currently clear if the infection can be passed on via fluids carried on the fingers, but kissing and sharing towels or cutlery won’t transfer the bacteria. Using a condom whenever you have sex will reduce the chance of transmission significantly.

Treating gonorrhoea involves an injection of antibiotics and a dose of antibiotic tablets, with follow up treatments two to four weeks later if needed. Visible symptoms should start to improve quickly, but you should avoid having sex until all symptoms are gone.


Another bacterial STI, Syphilis develops in three stages, but can be cured if treated early enough.

The symptoms across the three stages of the infection. During the first stage (primary syphilis), a small painless sore called a chancre may appear on the area where the infection entered the body, usually on the genitals or anus, but sometimes on the lips, tonsils, hands or buttocks. Lasting two to six weeks, the chancre is highly infectious. A few weeks after the chancre disappears, stage two (secondary syphilis) begins, with symptoms including a non-itchy rash on the palms or soles of the feet, small flat growth around the vagina or anus, flu-like symptoms, swollen glands in the neck, armpits or groin, weight loss, patchy hair loss, or white patches on the tongue. The symptoms may disappear and reappear over a period of weeks or months. The third and final phase is the tertiary stage, which is rare to get to in the UK. This can include skin rashes, numbness, paralysis, blindness, stroke, or heart disease to name a few, even causing death. [7]

The bacteria responsible for syphilis, called treponema pallidum, is passed on via unprotected penetrative or oral sex, or sharing sex toys. It can also be passed from mother to her unborn baby and should be treated during pregnancy to avoid birth complications. The antenatal screening includes a blood test that checks for syphilis.

Antibiotic injections are used to treat syphilis, which usually cures the infection in stages one or two of its development. Treatment may still be effective in the late stage of the infection, although severe bodily damage will be irreparable. Symptoms should improve quite quickly following treatment, but it’s important you do not have sex during this recovery period to prevent further transmission.

Pubic Lice

Also known as ‘crabs’, pubic lice are tiny parasitic insects that live on pubic hair.

The crab-like organisms are approximately 2mm and so can be difficult to see. They are yellow-grey or red in colour and multiply by laying eggs at the base of hairs, which are light brown coloured. You may not be able to see the public lice or their eggs, but you may be itchy in the affected area, see black powder in your underwear (lice droppings) or see small specks of blood or blue spots on the skin where the lice have bitten. Pubic lice can also appear on other coarse hair like the armpit, leg, back or facial hair, but not the head hair. [8]

Pubic lice are passed on through close bodily contact especially through penetrative or oral sex, but also through shared bedding or bed linen. Condoms are not effective at preventing the spread of public lice since they do not cover your pubic hair.

It’s important to treat pubic lice as soon as they’re noticed to avoid passing them on to others, since pubic lice won’t go away on their own. A topical cream such as Lyclear is effective at killing the lice and controlling the spread of the infection, also used to clear mites such as scabies. It should be applied to the entire body other than the head, face, throat and soft areas of the genitals. One application is usually enough to treat the STI sufficiently, but another can be done two weeks later if symptoms persist.

Toby Watson

Written by: Toby Watson

Pharmica Medical Writer

Toby (BSc) is an experienced medical writer, producing educational articles on many areas of health including sexual health, fitness, nutrition and mental health.

He particularly enjoys debunking misconceptions around heath conditions and their treatments, researching each topic in detail and writing easily-accessible content.

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