Pre-exposure prophylaxis, or PrEP, is a way of preventing HIV. If you’re HIV-negative, you can take certain HIV medications before coming into contact with HIV, and it reduces the likelihood that you will contract HIV.

PrEP is very very effective. If you take 7 pills a week, your risk of contracting HIV goes down by 99%. That is, if you receive unprotected anal sex from an HIV-positive partner, you have a 1 in 100 chance of getting HIV. If you are on PrEP, you instead have a 1 in 10000 chance of getting HIV. If you use condoms, you have a 1 in 50000 chance of getting HIV. [ETA: numbers fixed; thank you Keller.]

Even if you miss a pill sometimes, PrEP can be very effective. If you take four pills a week, your risk goes down by 96%, and if you take only two pills a week, your risk goes down by 76%.

You have to take PrEP for a week for it to start working for anal sex, and three weeks for it to start working for vaginal sex. When you stop taking PrEP, you should continue taking it for at least four weeks after your last possible exposure to HIV.

PrEP has very few side effects and is considerably safer than, for example, the birth control pill. Some people report gastrointestinal issues in the first month of taking it (I personally experienced nausea for one day), but these side effects will go away after the first month. If you take PrEP, you may experience a small increase in serum creatinine, which is filtered by the kidneys. While PrEP has not been linked to loss of kidney function, your doctor will monitor your kidney function, and it might not be a good idea to take it if you have kidney issues. If you’re on PrEP, you will have to go to the doctor more often for blood tests. People have taken PrEP for up to five years without long-term health consequences.

PrEP is usually covered by insurance, and there are medication assistance programs to help high-risk people pay for it.

PrEP only protects against HIV, not other STIs or pregnancy.

You should consider being on PrEP if you have sex that puts you at a high risk of contracting HIV. Here are some people who should consider being on PrEP:

People who have sex for money, drugs, housing, etc. and their partners.

People who inject drugs and share needles.

People who have unprotected sex with people who are HIV-positive or whose status they are not sure of. (“I met him that night and he said he was clean” does not count as “sure of.”)

People who drink heavily or use drugs in a context where they might wind up having unprotected sex.

People who have recently contracted an STI.

People who have anonymous sex with men or trans people who have sex with men or trans people.

It can be easy to overestimate your risk of HIV. For example, if you are polyamorous, have sex within your extended polycule, use condoms reliably, have a gossipy enough polycule that you know the HIV status and condom use habits of random acquaintances*, and know that everyone you’re sleeping with uses condoms reliably and gets tested regularly and doesn’t have HIV, you’re pretty safe from HIV. I don’t think people should rush out and get PrEP without considering how likely it is that they will get HIV.

However, even if you aren’t at high risk of HIV, you may want to consider taking PrEP for peace of mind. If you regularly feel anxious about contracting HIV or are avoiding casual sex that would make you happy and fulfilled because of fear of HIV, PrEP might be a good decision for you even if you’re objectively at fairly low risk.

*This may be redundant with ‘is a polycule.’