PEP, PrEP and U=U

PEP: after an exposure

Post-exposure prophylaxis is a short course of HIV medicines taken after a possible exposure to stop the virus from establishing an infection. You take it daily for 28 days.

How to get it

• Go to an emergency department, urgent care center, or sexual health clinic and say you need PEP.
• You'll be asked about the exposure and given a baseline HIV test. You don't need to wait for the result before starting.
• Take every dose for the full 28 days. Side effects with current regimens are usually mild.
• Expect follow-up HIV tests about 4 to 6 weeks and 12 weeks after the exposure.

Who it's for

PEP is recommended after a substantial exposure: sex without a condom or needle-sharing with someone who has HIV and is not virally suppressed, or with someone whose status is unknown but who may be at high risk. It's also used after sexual assault. A clinician will help you decide based on the details.

PrEP: before an exposure

Pre-exposure prophylaxis is medicine taken by people without HIV to prevent getting it. Taken as prescribed, it lowers the risk from sex by about 99%, and from injection drug use by at least 74%.

Options

• Daily pills. The most widely available option.
• Event-driven pills ("2-1-1"). Taken around sex, an option for some men who have sex with men.
• Injections. Long-acting injections given every two months, or twice a year with newer options.

Who might consider it

• Anyone whose sexual partner has HIV and isn't virally suppressed, or whose status is unknown
• People who don't always use condoms with new or casual partners
• Anyone diagnosed with an STI in the past six months
• People who inject drugs and share equipment
• Anyone who has used PEP more than once

PrEP needs regular HIV tests and check-ups. If you've just finished a course of PEP and expect more exposures, you can often move straight onto PrEP.

U=U: undetectable means untransmittable

When someone with HIV takes treatment and keeps their viral load undetectable (under 200 copies per milliliter), they do not pass HIV on through sex. Large studies of couples where one partner had HIV and one didn't found no transmissions from partners with a suppressed viral load, across many thousands of acts of sex without condoms.

It usually takes up to six months of treatment to reach an undetectable level, and staying there depends on taking treatment consistently. U=U applies to sex; it's not established for sharing needles, so needle-sharing still carries risk.

How these show up in your estimate

In the calculator, PrEP and PEP each reduce the estimate according to how consistently you took them and how soon PEP was started. A partner with an undetectable viral load sets the sexual risk to zero. The methodology pagelists the exact figures.