That depends on which medication you are taking for PrEP.

Truvada is associated with some weight loss

Truvada for PrEP does not cause people to gain weight. On the contrary, a small percentage, about 2.2%, of people taking Truvada for PrEP have reported some weight loss.

Descovy is associated with some weight gain

Descovy for PrEP is associated with weight gain. In the DISCOVER trial, researchers found that the average weight gain for people on Descovy was 2.2 pounds (1 kg) over the course of 48 weeks. "Average" means that some people gained less than 2.2 pounds (or did not gain any weight at all), but some people gained more than 2.2 pounds. In the same 48-week period, the average weight gain for people on Truvada was zero.