Skip to main content

Side effects of Pre-Exposure Prophylaxis (PrEP)

Like all medicines, PrEP can cause side effects, although not everyone gets them. Common side effects usually go away after the first few weeks.

Common side effects

These common side effects of PrEP may happen in more than 1 in 100 people. There are things you can do to help cope with them:

Feeling or being sick (nausea or vomiting)

Stick to simple meals and do not eat rich or spicy food. It might help to take your PrEP after you've eaten. Taking it just before bed can also help to avoid feeling sick.

If you really need to, you can take anti-sickness tablets. Ask a pharmacist which type is suitable for you.

If you're sick within 1 hour after taking PrEP, take another tablet. If you're sick more than 1 hour after taking PrEP, do not take another tablet.

If you're being sick, try small frequent sips of water to avoid dehydration. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee.

If you take contraceptive pills and you're being sick your contraception may not protect you from pregnancy. Check the pill packet for advice.

Bloating and indigestion

Try not to eat too much of foods like lentils, peas, beans and onions as they can cause wind. Eat smaller meals, eat and drink slowly, and exercise regularly.

There are pharmacy medicines that can also help, such as charcoal tablets or simeticone. You can also try peppermint oil capsules or drinking peppermint tea. You could also try taking an antacid for indigestion. Speak to a pharmacist for advice.


Drink plenty of water by having small frequent sips to avoid dehydration. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee. Do not take any other medicines to treat diarrhoea without speaking to a pharmacist or doctor.

If you get severe diarrhoea and it lasts more than a few days, contact your clinic for advice.

If you take contraceptive pills and you have severe diarrhoea for more than 24 hours, your contraception may not protect you from pregnancy. Check the pill packet for advice.


Make sure you rest and drink plenty of fluids. Do not drink too much alcohol.

You can take paracetamol or ibuprofen as long as you have not been told to avoid these. Tell your clinic if you are regularly taking ibuprofen or diclofenac for pain or headaches as these can affect your kidneys.

Feeling dizzy or weak

Stop what you're doing and sit or lie down until you feel better. Do not drive, cycle or use tools or machinery if you're feeling dizzy or weak. Do not drink alcohol as it may make you feel worse.

Insomnia (trouble sleeping)

If you're taking daily PrEP, take your tablet in the morning to avoid sleep problems at night.

If this advice does not help and the side effects bother you or do not go away, visit your clinic for advice.

Serious side effects

Very few people taking PrEP have serious problems.

PrEP can sometimes affect your kidneys. This is why kidney tests are done before and during treatment.

PrEP is more likely to affect your kidneys if you're 50 or above and already have kidney problems.

It can also affect your bone health, particularly if you have bone mass density loss (BMD), but this is rare. The risk also stops once you stop taking PrEP.

Serious allergic reaction

In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to PrEP.

Immediate action required: Call 999 now if:

  • your lips, mouth, throat or tongue suddenly become swollen
  • you're breathing very fast or struggling to breathe (you may become very wheezy or feel like you're choking or gasping for air)
  • your throat feels tight or you're struggling to swallow
  • your skin, tongue or lips turn blue, grey or pale (if you have black or brown skin, this may be easier to see on the palms of your hands or soles of your feet)
  • you suddenly become very confused, drowsy or dizzy
  • someone faints and cannot be woken up
  • a child is limp, floppy or not responding like they normally do (their head may fall to the side, backwards or forwards, or they may find it difficult to lift their head or focus on your face)

You or the person who's unwell may also have a rash that's swollen, raised, itchy, blistered or peeling.

These can be signs of a serious allergic reaction and may need immediate treatment in hospital.

Other side effects

These are not all the side effects of PrEP. For a full list, see the leaflet inside your medicine packet.


You can report any suspected side effect using the Yellow Card safety scheme.

Visit Yellow Card for further information.

Page last reviewed: 10 March 2023
Next review due: 10 March 2026