PrEP on-demand (2-1-1 dosing) is recommended for cisgender men who have sex with men. 
The 2-1-1 schedule is: 
2 pills 2-24 hours before sex 
1 pill 24 hours after the first dose 
1 pill 48 hours after the first dose 
It is not recommended for vaginal/front-hole sex because drug levels differ
 
This medicine is also good for the following situations 
-Daily PrEP for vaginal sex
-Daily PrEP for anal sex