Have been fisting for decades. I am always open and straightforward about it, especially when dealing with medical issues and treating doctors. They need to know what happens and what they need to look out for.
Story 1: First colonoscopy at 35 because my then proctologist was very aware that I fist (that's why I had the check). As I went under, he said - it's really a waste of anaesthetic. He doesn't need it ..
Story 2: My GP and I have a frank relationship about my fisting. I have a sigmoidoscopy every 6 months just to see all is ok. Once he said: "It's really easy doing this with you - absolutely no hassles and everything is wide open and easy to see ... and you are always superbly prepped "
Story 3 - Regular GP away, booked in for prostate check. Young female Dr who started the whole "relax" prep talk. I looked her straight in the eye and said - don't bother, you won't have any trouble with this examination. She asked me to lie on my side, gloved and lubed and there was a definite pause of surprise. "Yes, I see. I was told about this, but frankly didn't believe it". We chatted for a while about it.
Story 4. Had to have a fissure cauterised (too much embarassing blood after heavy sessions) and was in the ward for a week under observation and being fed intravenously so nothing irritated the healing. Young Asian Dr let it subtlely be known there was some empathy to my plight. "I know you have a regular doctor, but I think I would be able to give you a fuller and deeper examination." We examine each other very regularly now!
By being open about it, there is no pushback. Most of the medical staff I have dealt with are interested and sometimes the conversations are quite incredible.