HIV is a virus that invades the cells in your immune system, where it then multiplies and spreads throughout your body. PrEP works by protecting your immune cells and preventing HIV from getting inside them. When it can’t get inside your cells, the virus eventually dies.
Once you have started taking daily PrEP, it takes some time to fully protect your cells. It takes seven days for the drug to reach maximum levels in your rectal tissue, which means it will take one week to give you full protection for anal sex. It takes 21 days for PrEP to reach maximum levels in vaginal tissue. If you have a vagina, that means it will take three weeks to give you full protection for vaginal sex.
While you’re waiting for PrEP to become effective, using condoms for anal and vaginal sex will protect you from HIV.