What are the side-effects? Very few people who use PrEP encounter side effects – the most common can be headaches or feeling sick and these usually disappear after a few days. Less than 10% of people have reported mild nausea, mild bowel disturbance, bloating, and headaches. These side effects usually stop within the first month. Occasionally, PrEP can cause more serious side effects that reduce kidney function. Therefore you will have monitoring blood tests before and during the time you’re on PrEP.