PrEP and PEP are two medications strategies that prevent HIV infection, but they work on opposite timelines. PrEP (pre-exposure prophylaxis) is taken before any potential exposure to HIV, as ongoing prevention. PEP (post-exposure prophylaxis) is an emergency course of medication started after a possible exposure, within a strict 72-hour window. Both use antiretroviral drugs that block HIV from establishing itself in the body, but who they’re for, how long you take them, and how you access them are very different.
How PrEP Works
PrEP keeps a steady level of HIV-blocking medication in your body so that if the virus enters through sex or shared injection equipment, it can’t take hold and replicate. When taken as prescribed, PrEP reduces the risk of getting HIV from sex by about 99%. Among people who inject drugs, the risk reduction is at least 74%.
There are currently four FDA-approved PrEP options:
- Truvada (or generic equivalent): a daily pill
- Descovy (or generic equivalent): a daily pill
- Apretude: an injection given every two months
- Yeztugo: an injection given every six months
The injectable options are useful if taking a daily pill feels like a burden or if you’d prefer not to keep medication at home. With Apretude, you receive two starter injections one month apart, then continue with injections every two months. Yeztugo requires visits only twice a year. Both are administered by a healthcare provider. Before starting any form of PrEP, you’ll need a confirmed negative HIV test, and you’ll continue getting tested regularly while on it.
On-Demand PrEP
For people who don’t have frequent exposures, there’s a studied dosing pattern called “2-1-1” or event-based PrEP. You take two pills 2 to 24 hours before sex, one pill 24 hours after the first dose, and one more pill 24 hours after that. This approach uses the same daily oral medication but only around the time of a specific event, rather than every day. It has not been approved by the FDA as an official dosing schedule, but it has been studied and is used in some clinical settings outside the U.S.
Side Effects of PrEP
Most people tolerate PrEP well. The daily pill forms can cause small, usually insignificant decreases in kidney function that typically return to normal after stopping the medication. Bone density changes have also been observed, particularly in younger users (ages 15 to 19 saw larger declines than those 20 to 22). These effects are monitored through routine blood work: kidney function is checked at least once a year for all oral PrEP users, and more frequently if you’re over 50 or have conditions like diabetes or high blood pressure.
For the injectable forms, the most common side effect is a reaction at the injection site, including pain, tenderness, or mild swelling. These reactions are generally short-lived. Kidney monitoring isn’t required for injectable PrEP. Regardless of which form you use, you’ll have HIV testing at least every two to three months to confirm you remain negative.
How PEP Works
PEP is for emergencies. If you think you were exposed to HIV through unprotected sex, a needle stick, sexual assault, or shared injection equipment, PEP can prevent the virus from establishing an infection, but only if you start within 72 hours. The sooner you begin, the better it works. After 72 hours, PEP is no longer recommended because the window of effectiveness has closed.
PEP involves taking a combination of three antiretroviral medications every day for 28 days. That’s four full weeks with no missed doses. It’s a more intensive regimen than daily PrEP, and completing the entire course is critical. If you stop early, the drugs may not fully prevent the virus from taking hold.
PEP is not intended for repeated use. If you find yourself needing PEP more than once, that’s a strong signal that daily PrEP would be a better fit for your situation.
PrEP vs. PEP at a Glance
- Timing: PrEP is taken before exposure, on an ongoing basis. PEP is started after exposure, within 72 hours.
- Duration: PrEP continues as long as you want protection (daily pill or periodic injections). PEP is a fixed 28-day course.
- Purpose: PrEP is planned, long-term prevention. PEP is an emergency measure.
- Medications: PrEP uses one or two drugs (pill or injection). PEP uses a three-drug combination.
- Availability: PrEP is prescribed through a regular healthcare provider with routine lab work. PEP is often started in emergency rooms or urgent care settings because of the time-sensitive nature.
Cost and Access
Both PrEP and PEP can be expensive without insurance, but several programs exist to reduce or eliminate the cost. Each PrEP manufacturer runs a patient assistance program for people who are uninsured or whose insurance doesn’t cover the medication. These programs generally cover people with household incomes up to 500% of the federal poverty level. If you do have insurance but face high copays, co-payment assistance programs can offset up to $7,200 to $8,000 per year depending on the medication.
One important limitation: these programs typically cover only the medication itself. The cost of provider visits, lab tests, and HIV screenings that go along with PrEP or PEP are usually not included. Many community health centers and sexual health clinics offer these services on a sliding fee scale, which can fill that gap.
For PEP specifically, time pressure makes access trickier. If you need PEP on a weekend or holiday, an emergency room is often the fastest route. Some cities also have sexual health clinics or hotlines that can connect you with a provider quickly. The key constraint is always the same: treatment must begin within three days of exposure, and every hour counts.