If you are able to carry a douche or enema with you, you might want to at least do a squirt after you take a hit. Not only will that be painful, it can increase your risk for HIV and other sexually transmitted infections. If you have time, clean once, then wait about thirty minutes and clean again. Be less paranoid about poop. They will probably affect your stomach, especially if you have not eaten, and quite commonly will make you have a bowel movement. And not just any kind of bottom. If you choose to have sex on any drug or mild-altering substance — alcohol being one of them — know that substances can limit your ability to detect pain or know when your butt has had enough.
So far, the only drug approved for PrEP is Truvada, but more drugs are on the way.
If you have a healthy diet with plenty of fiber chances are your ass is ready to go right now. And, yes, I left the dirty underwear there. Incorporating a fiber supplement like Metamucil into your diet can help.