PrEP — the details

PrEP — the details

If you’re at risk of getting HIV and have considered your options, you might have looked into PrEP. This treatment can stop you from contracting the virus even if you’re exposed to it. Here we’ll take a look at what PrEP means and how it can help you.

Daniel Atkinson
Medically reviewed by
Daniel Atkinson, GP Clinical Lead
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Medically reviewed by
Dr Daniel Atkinson
GP Clinical Lead
on August 02, 2022.
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What is PrEP?

PrEP (also known as Pre-Exposure Prophylaxis) is a form of HIV prevention. It’s used by people who are at risk of being exposed to the HIV virus. It should also be used during pregnancy or breastfeeding when there is a risk of the mother contracting HIV, as it can be passed on to their child.

It works by stopping the virus from replicating inside your body and, as a result, prevents HIV from developing. It should only be taken if you’re at risk of being exposed to the virus. If you’ve already got HIV then it won’t be effective, and you should seek other available treatment options with your clinician.

PrEP contains two main active ingredients: Tenofovir and Emtricitabine. Sometimes PrEP is marketed as Truvada – which is the branded version of PrEP. There is also a generic version of the same treatment, which contains exactly the same ingredients and works in the same way.

You’ll usually need to take PrEP every day if you’re at a regular risk of being exposed to HIV, for example if your partner has HIV. It can also be taken ‘on-demand’ in situations where you’re going to have sex with someone who is HIV positive, or with someone whose HIV status you don’t know. On-demand PrEP is only available for men who have sex with other men at the moment, and it needs to be taken in a specific way.

If you use PrEP exactly as your clinician tells you to, then it’s very effective at preventing HIV from developing. It won’t work if you’ve already been exposed to the virus. In those cases, you might need to explore other options such as PEP (Post-Exposure Prophylaxis) medications instead.

When used as prescribed, PrEP reduces the risk of getting HIV from sex by about 99%. There’s less data about how effective PrEP is when it’s used by people who inject drugs, but it’s thought to be at least 74% effective at preventing HIV when it’s used for this.

PrEP is most effective when combined with other methods and treatments. If your partner is HIV positive, but receives treatment to reduce their viral load to undetectable levels, then the chances of transmission basically reduce to zero.

In a study of 888 couples who took part in the PARTNER 2 project , it was found that after 58,000 acts of penetrative sex without a condom between couples where one of them had undetectable levels of HIV, there were no incidences of HIV transmission at all. This means that if PrEP medications are used correctly they effectively provide complete protection against you catching HIV – often referred to as undetectable = untransmissable (U=U).

PrEP is designed for use if you’re at risk of catching HIV. If you’re in a relationship with someone who’s already HIV positive then you should take it on a daily basis, and get tested every three months to make sure that your HIV status is still negative.

You can also use PrEP on-demand if you know you’re going to have sex with someone who doesn’t know their HIV status, or in situations where you know the person you’re going to have sex with is HIV positive.

If you’re taking PrEP for either of these reasons, you should still practice safe sex and use barrier protection like condoms. PrEP will only protect you from HIV and not from other STIs.

If you regularly inject drugs with needles that could be shared or contaminated then you should also take PrEP every day, and get tested every three months.

If you’re already living with HIV, or think that you could’ve been exposed to HIV, then PrEP won’t be suitable for you to use. In these cases it’s better to talk to a clinician about your options. Treatments such as PEP (Post-Exposure Prophylaxis) might be a better option, as they can be taken after exposure to the virus.

You should always take PrEP as instructed by your clinician in order to make sure that it’s providing full protection, and that you’re using it safely.

There are a couple of different ways to take PrEP. The first is if you’re being prescribed PrEP for daily use and the second is if you’re having it prescribed for on-demand use.

For daily use:

  • One tablet a day should be taken with food.
  • Take it at the same time every day.
  • Different dosages might be prescribed depending on whether you’re an adolescent or an adult.

If you forget to take your PrEP:

  • If you remember within twelve hours, take it and carry on with your next dose as normal.
  • If it’s longer than twelve hours, skip the dose and continue with your next one at the right time the following day.

For on-demand use:

On-demand PrEP is basically as it suggests. This is where you only take it when you know you’ll have sex with someone who is HIV positive or their HIV status is uncertain. You’ll take fewer tablets over a shorter time frame – before and after sex.

There are some rules to stick to to make sure it works, but it’s highly effective in preventing HIV exposure for high-risk men who have sex with other men .

  • Take a double dose (two pills) 2-24 hours before sex.

Then continue as follows:

  • Take just one pill exactly 24 hours after your initial double dose.
  • Take one more pill another 24 hours after that.

One of the best ways of remembering this dosage is to think ‘2-1-1’.

However you’re taking PrEP, whether daily or on-demand, you’ll need to take a HIV test every three months to make sure you’re still testing HIV negative. In the event you test positive, you’ll need to stop taking PrEP and discuss other options with your clinician.

How long does PrEP take to work?

PrEP will reach it’s maximum protection at different rates depending on the kind of sex you’re having. For instance, if you’re receiving receptive anal sex (bottoming) then you’ll get maximum protection after seven days of continual use. If you’re receiving receptive vaginal sex then it can take up to 21 days of continuous daily use to reach the same protection.

At the moment there isn’t any clinical data available on protection for insertive anal sex (topping) or insertive vaginal sex.

Studies show that taking PrEP on-demand is very effective at stopping the spread of HIV. If you take the recommended four doses (2-1-1) in the preceding 24 hours before, and the following 48 hours after sex, you should get maximum protection against contracting HIV.

What about on-demand PrEP?

On-demand PrEP is the option to take if you don’t have a partner who is HIV positive, but you’re anticipating the possibility of having casual sex with people who could be.

It involves you taking two pills, 2-24 hours before sex, then a further pill 24 hours after, followed by one final dose 24 hours after that.

It’s highly effective at helping to prevent HIV infection in men who are at high risk, who have sex with other men. It offers a viable solution to taking PrEP every single day and gives protection against HIV infection. However, this method is only suitable for anal sex and not for vaginal or frontal sex.

Do I still need to use a condom with PrEP?

Yes, you’ll still need to use a condom with PrEP. As PrEP only provides protection against HIV, you’ll still need barrier protection if you sleep with someone who might have a different STI.

Barrier protections like this, whether it’s a male or female condom, can provide 98% safe protection against HIV and other STIs when used correctly.

PrEP isn’t a form of contraception, so you’ll still need to use a condom to protect against pregnancy, or consider the contraceptive pill if you’re female.

For most people, there are very few side effects from taking PrEP. Some of the more common ones when you first begin treatment are likely to be nausea, fatigue and dizziness. Once you get used to your dosage and taking the medication every day these should pass very quickly.

Descovy is a different type of PrEP to Truvada, and at the current time it’s only licensed in the USA. Descovy contains the active ingredients Tenofovir Alafenamide and Emtricitabine.

It’s usually recommended for adolescents and adults to take if they’re at high risk of getting HIV through sex. However, this is only for those people who have receptive or insertive anal sex and doesn’t include people who are at risk of getting HIV through vaginal sex. There are currently no studies into using Descovy for vaginal sex.

PEP stands for Post- Exposure Prophylaxis, whereas PrEP is Pre-Exposure Prophylaxis. You take PrEP to prevent you from getting HIV if you think you might be at risk of becoming exposed to it. PEP is only to be taken after you could’ve been exposed to the virus.

You’ll usually need to take Truvada post-exposure for 28 days after your possible exposure, but it’s always a good idea to talk with your prescribing clinician about your circumstances.

PEP should be taken within 72 hours of exposure, and is meant for emergency situations only. It isn’t as effective as PrEP is at preventing the spread of HIV. PrEP remains the best HIV prevention strategy for people who don’t have HIV but are at risk of contracting it.

How to get PrEP

PrEPPrEP is available free of charge throughout the UK — however it is easier to get in some regions than others. You can’t get PrEP from your usual GP, you’ll need to attend a specialist sexual health clinic in person if you want it on the NHS.

You can also get PrEP on a private prescription from many online pharmacies (like ours). This might suit you better if getting to a specialist sexual health clinic for PrEP isn't particularly convenient for you. All you'll need to do is fill out a quick consultation and your treatment, in many cases, can be sent straight to your door.

Just make sure that wherever you choose to get your treatment from is a properly regulated and registered pharmacy, compliant with both the General Pharmaceutical Council (GPhC) and the Care Quality Commision (CQC).  

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This page was medically reviewed by Dr Daniel Atkinson, GP Clinical Lead on August 02, 2022. Next review due on August 01, 2024.

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When we present you with stats, data, opinion or a consensus, we’ll tell you where this came from. And we’ll only present data as clinically reliable if it’s come from a reputable source, such as a state or government-funded health body, a peer-reviewed medical journal, or a recognised analytics or data body. Read more in our editorial policy.

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