PEP Medication After a Possible HIV Exposure: The 72-Hour Window, the 28-Day Course and Side Effects

If you think you were exposed to HIV in the last 72 hours, stop reading and get help now. In the US, go to an emergency room or urgent care clinic, or call your healthcare provider. In the UK, go to a sexual health clinic or A&E. In Australia, use the Get PEP clinic finder¹²³. PEP works best the sooner it starts, and ordering medicine online is far too slow.

The rest of this guide explains what PEP is, what the 28 days involve, and what happens afterwards.

What PEP is

PEP stands for post-exposure prophylaxis. It is a 28-day course of daily HIV medicines, started very soon after a possible exposure, to stop the virus from taking hold in the body¹.

PEP may be right for you if you are HIV-negative, or do not know your status, and in the last 72 hours you¹:

• had sex without a condom, or a condom broke, with a partner who has HIV and is not known to be virally suppressed, or whose HIV status you do not know, and you were not using PrEP
• shared needles, syringes or other equipment for injecting drugs
• were sexually assaulted
• had a possible exposure at work, such as a needlestick injury

US guidelines recommend PEP when the partner with HIV is not known to have a sustained undetectable viral load⁴. People with HIV who keep an undetectable viral load do not pass HIV on through sex⁶. Our guide to undetectable viral load explains why.

The 72-hour window, and why sooner is better

US guidelines say the first dose should be given as soon as possible, ideally within 24 hours, and no later than 72 hours after the exposure⁴. HIV.gov puts it simply: every hour counts¹.

What if it has been more than 72 hours? The 2025 CDC guidelines say there is not enough evidence to recommend starting PEP that late. People who come forward after 72 hours should still be tested for HIV, offered advice on prevention including PrEP, and given a plan for follow-up testing⁴. In the UK, the NHS advises going to a sexual health clinic or GP for testing if the exposure was more than 72 hours ago².

Where to get PEP quickly

• United States: an emergency room, urgent care clinic or your own healthcare provider. HIV.gov’s locator lists PEP services, and pharmacists can prescribe PEP in some states, including California, Colorado, Illinois, New York, Oregon and Virginia¹.
• United Kingdom: sexual health clinics and A&E².
• Australia: the Get PEP website lists clinics, including some that offer PEP at a reduced cost for people without Medicare³.

Before PEP starts, you will usually have a rapid HIV test. The first dose should not be held up while waiting for other lab results⁴. US guidelines encourage services to give the first dose at the first visit and send you home with a starter pack or the full course⁴.

What medicines are used for PEP

PEP uses the same kinds of medicines used to treat HIV. The 2025 CDC guidelines list two preferred options for adults and teenagers, each taken once a day for 28 days⁴:

• Bictegravir with emtricitabine and tenofovir alafenamide, the three medicines in Biktarvy, as one tablet.
• Dolutegravir plus a tenofovir (tenofovir alafenamide or tenofovir disoproxil fumarate) plus emtricitabine or lamivudine. In practice this is often dolutegravir with a tenofovir and emtricitabine tablet.

A boosted darunavir regimen is listed as an alternative. If the source partner is known to have drug-resistant HIV, the doctor may choose different medicines⁴. Guidelines in other countries may use different combinations, so the medicines you are given can differ from these.

For more on the individual medicines, see our list of HIV medication names and Biktarvy explained.

The 28 days: side effects and missed doses

The preferred PEP medicines are generally safe and well tolerated⁴. Some people have nausea, and less often other side effects. In almost all cases these can be treated and are not dangerous¹. The side effects most often listed for Biktarvy, for example, are diarrhoea, nausea and headache⁵.

A few things help you get through the course:

• Take every dose. PEP is most effective when all doses are taken as prescribed¹. Set a daily alarm.
• If you miss a dose, follow the leaflet for your medicine and do not double up without advice. Our guide to missed doses of HIV medication covers the rules for Biktarvy and dolutegravir.
• Tell the prescriber about everything else you take, including over-the-counter medicines, supplements, alcohol and recreational drugs, because PEP medicines can interact with them¹. Antacids and iron or calcium supplements need spacing from integrase inhibitors; see our guide to HIV medicine interactions.
• If a side effect bothers you, contact your clinic rather than stopping. Something can usually be done.
• Use condoms and sterile injecting equipment during the course. PEP is not protection against new exposures while you take it¹.

PEP can be used in pregnancy and while breastfeeding; your provider will choose the regimen¹.

Testing during and after PEP

US guidelines recommend a check-in about 24 hours after starting (in person or remotely) and follow-up visits with HIV testing at 4 to 6 weeks and at 12 weeks after the exposure⁴. Your clinic may also test for other sexually transmitted infections⁴.

These follow-up tests matter. PEP works well when started early and taken properly, but it does not work every time¹⁴. HIV medicines taken as PEP can also delay a positive result, which is why the guidelines recommend specific tests at follow-up⁴.

After PEP: moving on to PrEP

PEP is for emergencies. It is not meant for regular use by people who may be exposed often¹. If you are likely to be exposed again, ask about PrEP (pre-exposure prophylaxis), which is taken before exposure. The CDC asks clinicians to offer PrEP to people with ongoing risk and to plan a direct switch from PEP to PrEP⁴.

Our guide to PrEP vs PEP explains the difference, and daily vs on-demand PrEP covers the two ways to take PrEP pills. For starting and stopping PrEP safely, see how long PrEP takes to work. Our PrEP guide covers generic PrEP such as Tenvir-EM (emtricitabine with tenofovir disoproxil fumarate).

Frequently asked questions

Does PEP always work?

No. PEP is effective when started quickly and taken correctly for the full 28 days, but it does not work every time. That is why you need follow-up HIV tests at 4 to 6 weeks and 12 weeks, and why starting as early as possible matters.

Can I take PEP after 72 hours?

Current US guidelines say there is not enough evidence to recommend starting PEP after 72 hours. If more than 72 hours have passed, you should still get an HIV test, advice on prevention, and a plan for follow-up testing.

Can Truvada be used as PEP?

Truvada contains emtricitabine and tenofovir disoproxil fumarate, which form part of some PEP regimens, but on its own it is not a complete PEP regimen. Current guidelines combine it with a third medicine, such as dolutegravir.

Can PEP cause HIV?

No. PEP medicines are antiviral medicines and do not contain the virus. If someone tests positive after PEP, the HIV came from the exposure, not from the medicine.

Can I drink alcohol on PEP?

The Biktarvy label does not list alcohol as an interaction, but tell your prescriber how much you drink. Heavy drinking makes missed doses more likely, and the full 28 days matter.

Can I buy PEP online?

For an exposure that has just happened, no: online delivery is too slow. PEP should start within hours, so go to an emergency service or clinic. Online pharmacies are better suited to ongoing PrEP.

Sources & references

1. HIV.gov. Post-Exposure Prophylaxis (PEP). Updated 8 September 2026.
2. NHS. HIV and AIDS: prevention. Last reviewed 9 July 2025.
3. Get PEP (Australia). Find PEP near you.
4. Tanner MR, et al. Antiretroviral Postexposure Prophylaxis After Sexual, Injection Drug Use, or Other Nonoccupational Exposure to HIV: CDC Recommendations, United States, 2025. MMWR Recommendations and Reports. 8 May 2025;74(1).
5. US National Library of Medicine, DailyMed. Biktarvy prescribing information.
6. HIV.gov. HIV Treatment as Prevention.

Related reading

• PrEP vs PEP
• PrEP: daily vs on-demand
• Dolutegravir medicines and emtricitabine medicines
• All HIV and PrEP medicines

This content is for informational purposes only and is not medical advice. PEP is an emergency treatment: if you may have been exposed to HIV, contact an emergency service or sexual health clinic straight away. Used as prescribed by a licensed medical professional.