Pre-Exposure Prophylaxis (PrEP)

What is pre-exposure prophylaxis?

Pre-Exposure Prophylaxis, or PrEP, is a medication taken by people who do not have HIV to reduce their risk of getting it. It contains two antiretroviral medicines: tenofovir disoproxil and emtricitabine, which are also used to manage HIV. When taken correctly, PrEP is highly effective at preventing HIV infection. It is available on prescription only[1]. 

What is pre-exposure prophylaxis used for?

PrEP is not used to treat HIV. Instead, it is used to prevent HIV infection in people who are HIV-negative but at high risk of exposure to the virus[1]. 

How does pre-exposure prophylaxis work?

PrEP works by stopping HIV from entering your cells and replicating. If HIV cannot replicate, it cannot establish an infection in your body[1]. 

Who can take pre-exposure prophylaxis?

PrEP is recommended for people who are considered at higher risk of acquiring HIV [1]. This includes[1]: 

• Men who have sex with men (MSM) and have condomless sex with multiple partners 

• People with a HIV-positive partner who is not on effective treatment 

• People who have sex with partners from countries where HIV is common 

• Trans and non-binary people having condomless sex with multiple partners 

• People who exchange sex for money, drugs, or shelter 

• People who inject drugs and share injecting equipment 

You should speak to your doctor first if you[1]: 

• Have ever had an allergic reaction to tenofovir disoproxil, emtricitabine, or any other medicine 

• Have severe liver or kidney problems 

• Are pregnant or breastfeeding 

• Have a weakened immune system 

• Are HIV positive 

• Have hepatitis 

You may not need to take PrEP if[1]: 

• Your partner is HIV-positive, takes HIV medicine, and has an undetectable viral load 

• You always wear condoms when you’re having sex 

Please tell your doctor or pharmacist about any conditions you may have or medication you take before you start treatment, and always follow the instructions in the Patient Information Leaflet[2][3].  

What is the typical dosage of pre-exposure prophylaxis?

PrEP can be taken in two different ways depending on your needs and the kind of sex you have[1]. 

For daily PrEP (suitable for all types of sex)[1]:

• One tablet should be taken at the same time each day, ideally with or just after food. You should wait 7 days after initiation before having sex 

• If you expect to have sex within the first 7 days, take two tablets at least two hours before. After this, continue with one tablet each day. 

• If you vomit within one hour of taking your dose, you will need to take another. 

• If you miss a dose, take as soon as you remember unless it is after 12 hours. If this is the case, skip the missed dose and continue as normal. 

• If you wish to stop taking the medication, continue taking for 7 days (for vaginal/frontal sex) or 2 days (for anal sex) after your last risk of exposure. It is then ok to stop 

• For on-demand PrEP (for anal sex only)[1]: 

• Take two tablets 2 to 24 hours before sexual activity is anticipated 

• After 24 hours of taking the first dose, take one tablet 

• After a further 24 hours take the final dose of one tablet 

• If you continue to have sex over a period of time, continue to take one tablet every 24 hours 

• Continue to take the medication for 48 hours after your last risk exposure

How do you take pre-exposure prophylaxis?

When taking PrEP, follow these tips to reduce side effects and ensure effectiveness[1]:

• Take with or just after food to reduce stomach upset 

• If you vomit within 1 hour of taking it, take another dose 

• If vomiting occurs after 1 hour, there’s no need to retake 

• If you struggle to swallow the medication, it may be crushed and mixed with water, orange juice or grapefruit juice. If doing this, ensure the whole tablet has been taken 

Can you stop taking pre-exposure prophylaxis?

Yes, you can stop taking PrEP if you are no longer at risk of HIV[1].

• For vaginal or frontal sex, continue taking PrEP for 7 days after your last risk exposure 

• For anal sex, continue taking it for 2 days after your last risk exposure 

What do you do if you take too much pre-exposure prophylaxis?

You should not take more than 7 tablets per week, unless you started with a double initiation dose. Taking more PrEP than prescribed is unlikely to cause serious harm, but it’s best to contact your clinic or NHS 111 for advice[1]. 

What do you do if you forget to take pre-exposure prophylaxis?

Here’s what to do if you miss a dose[1]: 

• If you remember within 12 hours, take the missed dose 

• If more than 12 hours have passed, skip it and take your next dose as usual 

• Missing more than 2 doses in a week reduces your protection – contact your clinic for advice 

If you have been taking PrEP for 7 days, you are still protected, even if you miss the occasional dose. If you have been taking PrEP daily, you must have taken at least 6 doses in the past week to still be protected[1].

How do you store pre-exposure prophylaxis?

To keep your medication safe and effective[1]:

• Store in a cool, dry place below 30°C 

• Keep in the original packaging 

• Keep out of reach of children

What medicines can’t you take with pre-exposure prophylaxis?

Some medicines can affect how PrEP works, particularly those that affect your kidneys. Always inform your GP, pharmacist or clinic if you're taking any of the following[1]: 

• Other prescription medications 

• Over-the-counter treatments 

• NSAIDs like ibuprofen or diclofenac 

• Hepatitis C medicine like sofosbuvir or ledipasvir 

• Herbal remedies or supplements 

• Fitness supplements like creatine 

• Gender-affirming hormones like HRT 

Always inform your doctor or pharmacist about all medications and supplements you take before you start a new medicine, and follow the instructions in the Patient Information Leaflet[2][3].

What are the side effects of pre-exposure prophylaxis?

Most people do not experience serious side effects. Common side effects, usually mild and temporary, include[1]: 

• Nausea 

• Diarrhoea 

• Bloating 

• Headache 

• Tiredness 

• Trouble sleeping 

Less common side effects include[1]: 

• Changes in kidney function (monitored through regular tests) 

• Slight reductions in bone density (rare and usually reversible) 

You should contact your GP if you experience any of these symptoms. In rare cases, the medicine may cause a serious allergic reaction (anaphylaxis). You should call 999 or visit A&E immediately if you believe this is happening[1]. 

Symptoms of anaphylaxis include[1]:  

• Difficulty breathing  

• Tightness in throat and chest  

• Swelling of the tongue or throat  

• Feeling faint or dizzy  

• Blue/grey skin or lips  

• Loss of consciousness 

If you experience any side effects, including those not listed, you are encouraged to report them via the Yellow Card Scheme or contact your doctor, pharmacist, or nurse. This information is intended as a summary for general awareness and does not replace professional medical advice. 

The information provided in the side effects section is not a comprehensive list of all possible side effects associated with this medication. For a complete list of side effects, please refer to the Patient Information Leaflet (PIL) provided with your medication or consult your healthcare professional[2][3].

Frequently asked questions about pre-exposure prophylaxis

How effective is PrEP? 
PrEP is around 99% effective when taken correctly[1]. 

Does PrEP protect against other STIs? 
No, PrEP only protects against HIV. It does not prevent other sexually transmitted infections[1]. 

Can I drink alcohol while on PrEP? 
Yes, alcohol does not interfere with how PrEP works[1]. 

Can I take PrEP with contraception? 
Yes, PrEP can be safely taken alongside any form of contraception[1]. 

What if I vomit or have diarrhoea? 
If you vomit within 1 hour of taking a dose, take another tablet. If vomiting happens after an hour, no need to retake[1]. 

Can I drive while on PrEP? 
Some people feel dizzy when they first start taking PrEP. If this affects you, avoid driving until it passes[1]. 

How long does it take for PrEP to work? 

• For vaginal or frontal sex: 7 days of daily dosing 

• For anal sex: 2 hours after taking a double dose 

Can I take PrEP if I have hepatitis B? 
Possibly. Talk to your clinic, as stopping PrEP can reactivate hepatitis B[1]. 

Can I crush the tablets? 
Yes. If you struggle to swallow tablets, they can be crushed and mixed with water or orange juice. Ensure the full dose is taken, an no medication is left in the glass[1]. 

For the official NHS guide to pre-exposure prophylaxis (PrEP), visit their website. 

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