PrEP and PEP sound almost the same, and both use HIV medicines to stop HIV from taking hold. The difference is timing. PrEP is taken before a possible exposure, as a regular habit. PEP is taken after one, as an emergency. Getting them mixed up can cost precious hours, so here is the difference in plain terms.
The one-line difference
- PrEP (pre-exposure prophylaxis): medicine taken by HIV-negative people who may be exposed to HIV, to prevent infection from sex or injecting drugs1.
- PEP (post-exposure prophylaxis): a 28-day course of HIV medicines started as soon as possible, and within 72 hours, after a possible exposure2.
If you think you were exposed in the last 72 hours and you were not on PrEP, you need PEP, now. Our guide to PEP after a possible HIV exposure explains where to get it.
PrEP and PEP side by side
| PrEP | PEP | |
|---|---|---|
| When | Before possible exposure, as ongoing protection | After a possible exposure: ideally within 24 hours, no later than 72 hours3 |
| How long | As long as you may be exposed | 28 days3 |
| Medicines | Two pills (Truvada, Descovy) or two injections (Apretude, Yeztugo)1 | Usually three medicines, such as the ingredients of Biktarvy, or dolutegravir with a tenofovir and emtricitabine3 |
| Tests | HIV test before starting, then every 3 months on pills (every 2 or 6 months on injections)1 | Rapid HIV test at the start, then follow-up at 4 to 6 weeks and 12 weeks3 |
| Where to get it | A doctor, sexual health clinic, health centre, or a pharmacist in some US states1 | Emergency services, urgent care and sexual health clinics, fast24 |
| Meant for | Repeated or ongoing risk | Emergencies, not regular use2 |
Which pills are approved as PrEP
In the US, four medicines are approved for PrEP1:
- Truvada (emtricitabine with tenofovir disoproxil fumarate), a daily pill for anyone at risk through sex or injecting drugs. Generic versions are available.
- Descovy (emtricitabine with tenofovir alafenamide), a daily pill for sexually active men at risk of HIV. It is not for women at risk through receptive vaginal sex.
- Apretude (cabotegravir), an injection every other month.
- Yeztugo (lenacapavir), an injection twice a year, started with tablets for the first two days.
Our guide to Truvada vs Descovy explains the two pills, and our guide to injectable PrEP compares the shots with daily tablets.
Why Biktarvy, Dovato or Triumeq is not PrEP
People often ask whether an HIV treatment pill they already have, or can get, could work as PrEP. Biktarvy, Dovato and Triumeq are approved to treat HIV56, and none of them is on the list of medicines approved for PrEP1.
There is also a safety reason to use PrEP through a proper PrEP service rather than a treatment pill taken on your own. PrEP always starts with an HIV test, and repeats it regularly, because taking HIV medicines while an infection goes unnoticed can lead to drug resistance. The World Health Organization reports that resistance to tenofovir is more than 10 times more common when PrEP is started during an undiagnosed new HIV infection7.
Some of these treatment medicines are, however, used in PEP, as part of a supervised 28-day course after an exposure3. That is a different job, with different tests and follow-up.
Can I take PrEP after an exposure?
Not as a substitute for PEP. Once a possible exposure has happened and you were not already protected by PrEP, the right medicine is PEP, started as soon as possible2. Taking a PrEP pill after the event is not the same thing.
On-demand PrEP, sometimes called 2-1-1, can look like an “after” schedule because doses continue after sex, but it starts with a double dose taken before sex. It is only suitable for certain people and certain types of sex. Our guide to daily vs on-demand PrEP explains who it suits.
Moving from PEP to PrEP
If you have needed PEP and you are likely to be exposed again, PrEP is worth discussing. HIV.gov lists using PEP, especially more than once, as one of the reasons PrEP may benefit you1. US guidelines ask clinicians to offer PrEP to people with ongoing risk and to plan a direct move from PEP to PrEP, so there is no gap in protection3.
For starting PrEP well, and for how long it takes to work, see how long PrEP takes to work.
Where doxy-PEP fits
Doxy-PEP is a different idea with a similar name. It is a single dose of the antibiotic doxycycline taken within 72 hours after sex to lower the chance of some bacterial sexually transmitted infections, such as syphilis and chlamydia. US guidelines say it might be considered for gay, bisexual and other men who have sex with men as part of wider STI care3. It does not protect against HIV. Our guide to doxy-PEP explains it in full.
Side effects compared
Both are generally well tolerated. HIV.gov describes PrEP as safe, with no significant health effects seen in HIV-negative people who took it for up to 5 years. Some people have nausea, diarrhoea, headache, tiredness or stomach pain, which usually settle over time1. On PrEP pills, your clinic also checks kidney function from time to time.
PEP uses three medicines for 28 days. Nausea is the side effect people notice most, and in almost all cases side effects can be treated and are not dangerous2. The preferred PEP medicines are generally safe and well tolerated3.
Cost and access
- United States. Under the Affordable Care Act, PrEP medicine, clinic visits and the lab tests needed for it are covered without co-payments by almost all health insurance plans, and by Medicaid expansion plans1. PEP is given through emergency rooms, urgent care and clinics.
- United Kingdom. PrEP is free on the NHS from sexual health clinics, and PEP is available from sexual health clinics and A&E4.
- Elsewhere. Generic PrEP pills are widely made and used. Our guide to what PrEP costs compares brand and generic prices.
Choosing: ongoing risk or a one-off?
- A one-off exposure, such as a condom breaking or a sexual assault: PEP, as fast as possible.
- Repeated or ongoing risk, such as a partner with HIV who is not virally suppressed, condomless sex with partners of unknown status, a recent sexually transmitted infection, or sharing injecting equipment: PrEP1.
- Both: PEP now, then straight on to PrEP.
Our PrEP guide covers generic PrEP pills such as Tenvir-EM (the same ingredients as Truvada) and Tafmune-EM (the same ingredients as Descovy), and the rules on prescriptions and import.
Frequently asked questions
Can Biktarvy be used for PrEP?
Biktarvy is approved for treating HIV, not for PrEP, and it is not on the list of approved PrEP medicines. Its ingredients are, however, one of the preferred options for PEP after an exposure, under medical supervision.
Is Truvada PrEP or PEP?
Truvada is approved as PrEP. Its two ingredients are also used in many PEP regimens, but for PEP they are combined with a third medicine, such as dolutegravir.
Can I take PrEP after an exposure?
No. If you were not already on PrEP, an exposure in the last 72 hours needs PEP, started as soon as possible at an emergency service or clinic.
What is the difference between PrEP and PEP medicines?
PrEP uses two-drug pills (Truvada or Descovy) or long-acting injections (Apretude or Yeztugo). PEP usually uses three HIV medicines for 28 days, such as bictegravir, emtricitabine and tenofovir alafenamide.
Do I need PEP if I am on PrEP?
If you take PrEP as prescribed, you generally do not. If you have missed doses, or started PrEP very recently, speak to a clinic straight away about whether PEP is needed.
Is PEP the same as the morning-after pill?
No. PEP is a 28-day course of HIV medicines to prevent HIV infection after an exposure. It does not prevent pregnancy, and emergency contraception does not prevent HIV.
Sources & references
- HIV.gov. Pre-Exposure Prophylaxis (PrEP). Updated 9 October 2026.
- HIV.gov. Post-Exposure Prophylaxis (PEP). Updated 8 September 2026.
- 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).
- NHS. HIV and AIDS: prevention. Last reviewed 9 July 2025.
- US National Library of Medicine, DailyMed. Biktarvy prescribing information.
- US National Library of Medicine, DailyMed. Dovato prescribing information.
- World Health Organization. HIV drug resistance (fact sheet). 27 July 2026.
Related reading
- How to get PrEP
- Doxy-PEP for STI prevention
- PEP after a possible HIV exposure
- Truvada vs Descovy
- Dovato vs Biktarvy vs Triumeq
- Generic Truvada and generic Descovy
This content is for informational purposes only and is not medical advice. If you may have been exposed to HIV in the last 72 hours, contact an emergency service or sexual health clinic straight away. Used as prescribed by a licensed medical professional.
Jacob T. is a licensed clinical pharmacist with 18 years of experience. He holds a B.Pharm and M.Pharm from Dr. M.G.R. Medical University. At OnlineMeds, he reviews medicine and health content to help ensure it is accurate, evidence-based, and easy to understand.