It is not meant to be used instead of a condom. It is meant to be used in conjunction with a condom. PrEP is NOT an emergency treatment. It is a daily preventive medication taken by people who are at higher risk for HIV - for example, men who have anal sex with multiple other men. It is a back-up in case the condom fails.
PrEP needs to be prescribed by a doctor, and the doctor will likely do periodic lab tests to ensure that the medication is not causing dangerous side effects.