HIV can pass through the inner lining of your genitals or anus during sex. Once inside, it attacks your white blood cells, reproducing and spreading quickly to the rest of your body. When you take PrEP, the medicine stops the HIV virus before it has a chance to get established inside your body.
We know from studies that PrEP is highly effective at stopping you from getting HIV. These were studies done amongst people at high risk of getting HIV, including gay and bi men who had anal sex without condoms. The studies compared those taking PrEP with those who took a dummy pill, or no medication at all. Those who didn’t take PrEP got HIV frequently, but those who did take PrEP hardly ever got HIV. The studies also showed that the small number of people who did get HIV after being given PrEP, either hadn’t taken PrEP consistently or had picked up HIV without realising just before they started PrEP. You can read more about those studies here.
The medicines in PrEP are also used to treat HIV, but only when used in combination with another antiretroviral medication. PrEP is brilliant for prevention, but not powerful enough to treat HIV by itself. If you take PrEP while you have HIV without knowing, the HIV virus may become resistant to the medicines in PrEP making the HIV more difficult to treat. For this reason it’s essential to do a HIV test just before you start PrEP, and to keep testing regularly when you’re on PrEP, at least every 3 months.