Hi doctor, my girlfriend wants to perform anal sex on me but i have ulcerative colitis, i'm in remission but is it safe?

Still use precaution: Should still use the usual precautions as per standard (condoms, dental dam etc). May be uncomfortable and you need to be very sure you are in remission/have that verified by your doctor otherwise you may have some contact bleeding and be at increased risk of other injuries. I suggest a consultation with your GP who has your records prior to your sexual activity.

Here are some...: Do you want to get a professional blessing for anal sex? In reality, no one can offer such; anal sex is known to be a factor increasing risk for STD or unwanted hurting. The only way to avoid unwanted consequence is do not do it. So, you decide your own behavior as long as being willing to be responsible for what you do. In general, I recommend to avoid it; yet, you decide.