PrEP vs PEP — HIV Prevention Explained

If you are trying to understand the difference between PrEP and PEP, you are likely either thinking about ongoing HIV prevention or dealing with a recent possible exposure. The short version:

• PrEP (pre-exposure prophylaxis) is a daily (or now bi-monthly) medication taken before any exposure to prevent HIV.
• PEP (post-exposure prophylaxis) is a 28-day course of HIV medication started within 72 hours after a possible exposure as emergency treatment.

Different drugs in some cases. Different timing. Different scenarios. Here is the full picture.

PrEP — daily prevention for ongoing risk

PrEP is for people who are HIV-negative and have ongoing situations that could expose them to HIV. The most common candidates:

• People with HIV-positive sex partners who are not yet undetectable
• Men who have sex with men with multiple or anonymous partners
• People in serodiscordant relationships where condoms are not always used
• People who inject drugs
• Transgender women
• People in geographic areas with high local HIV prevalence

Current PrEP options

Daily oral PrEP (the original):

• Truvada (tenofovir disoproxil fumarate + emtricitabine) — generic available, cheapest
• Descovy (tenofovir alafenamide + emtricitabine) — newer, gentler on kidneys and bones, more expensive

One pill daily. Need a baseline HIV test before starting. Need to test every 3 months while on PrEP.

Long-acting injectable PrEP:

• Apretude (cabotegravir) — intramuscular injection every 2 months in the buttock or thigh

No daily pill. For people who struggle with adherence, or who simply prefer not to think about it.

Twice-yearly injectable PrEP:

• Lenacapavir (formerly trialed as PURPOSE-1 / PURPOSE-2) — subcutaneous injection every 6 months

Lenacapavir for PrEP was approved by the FDA in 2024 after the PURPOSE trials showed near-100% protection in women in sub-Saharan Africa and high efficacy in MSM. Twice-yearly injection. This is poised to be transformational for global HIV prevention.

Effectiveness

Daily oral PrEP taken consistently is approximately 99% effective against sexual transmission of HIV.

Apretude is at least as effective as daily oral, possibly slightly more (because adherence is built in).

Lenacapavir is approaching 100% effective in the trials, with the caveat that real-world use will show its true number.

Common questions about PrEP

Does PrEP protect against other STIs? No. It only prevents HIV. Routine STI testing every 3-6 months is part of PrEP follow-up for a reason.

What about condoms? PrEP works without condoms. Condoms still protect against other STIs and unwanted pregnancy. Many people on PrEP use condoms situationally, not always.

Will I have to stay on PrEP forever? No. PrEP is on-demand life-circumstance medication. Take it during periods when you need protection; come off when you don't. If you stop, talk to your provider about timing — the tail-end of protection lasts about 2 days.

What about side effects? Most people have minimal side effects. About 10% have mild GI upset in the first few weeks. Kidney function is monitored quarterly (rarely an issue with Descovy or Apretude).

Insurance / cost? Truvada is generic and very cheap. Branded options have manufacturer copay programs. The Ready, Set, PrEP program in the US provides PrEP at no cost for people without insurance.

PEP — emergency post-exposure prophylaxis

PEP is for situations where someone has had a possible HIV exposure in the past 72 hours. The window is strict: PEP is only effective if started within 72 hours of exposure, and earlier is much better (within 24 hours is ideal).

When to consider PEP

• Condom break with a partner of unknown or positive HIV status
• Sexual assault
• Needle-stick injury (healthcare worker or person who injects drugs)
• Sharing injection equipment with someone of unknown or positive HIV status

If you might need PEP, do not wait — go to an emergency room, urgent care clinic, or sexual-health clinic. PEP is time-sensitive.

What PEP involves

A 28-day course of HIV antiretroviral therapy, typically:

• Tenofovir + emtricitabine + dolutegravir (or raltegravir)

You take it daily for 28 days. Side effects can include nausea, fatigue, and headache; they are usually manageable.

Follow-up:

• HIV test at the start (must be negative)
• HIV test at 6 weeks
• HIV test at 12 weeks
• HIV test at 6 months (for high-risk exposures)

If the source person can be tested, that helps direct decisions.

Effectiveness

Started promptly and completed fully, PEP reduces HIV acquisition risk by about 80%. It is not a guarantee — which is why prevention via PrEP is preferred when ongoing risk is known.

Cost / insurance

Most U.S. insurance plans cover PEP under ACA preventive-care rules. For uninsured patients, manufacturer programs (Gilead's Advancing Access for tenofovir-based regimens) and emergency-department social work can help.

When to switch from PEP to PrEP

If you completed a 28-day PEP course and the testing shows you remained HIV-negative, the next question is whether you have ongoing risk. If yes, transitioning directly to PrEP is reasonable and increasingly recommended. There is no medical gap needed between PEP and PrEP.

If you are on PEP because of a single isolated exposure (like a one-time condom break, or a sexual assault) and you don't have ongoing risk, you can stop after the 28-day course.

Side-by-side comparison

The bottom line

PrEP is the right tool when you know you have ongoing exposure risk and want sustained protection. PEP is the right tool when something happened, you are inside the 72-hour window, and you need emergency intervention.

Both are extraordinarily effective. Both are widely accessible in the US, often at no cost. Both should be in the prevention toolkit of anyone whose sexual or drug-use situation puts them at risk.

If you had an exposure in the past 72 hours, stop reading and find a PEP source. If you have ongoing risk, ask your provider about PrEP at your next visit — or use a telehealth service that specializes in it.

For more on HIV — testing, U=U, current treatments, cure research, and personal stories from people living with HIV — see our complete HIV pillar guide.