If your gf isnt already completely comfortable with it, i wouldnt start on pills.
1) a very good point was raised about pain receptors and if you yourself arent familiar with anal sex, id suggest you have some practice before going at it with no idea how much damage will be felt the next day
2) you dont want her to get upset part way through
3) you dont want her to get upset with you afterwards...next day or whatever...as she deals with her embarrassment.
you should get her comfortable with ideas like this when she's feeling normal. doing drugs with her to make her relax might make for a difficult navigatory path afterwards for both of you. if she isnt completely relaxed about doing this sort of thing then its going to be damned hard for you to actually get it in and its going to hurt her. muscles need to be relaxed.
4) if she isnt completely relaxed about doing this sort of thing then its going to be damned hard for you to actually get it in and its going to hurt her. muscles need to be relaxed.
5) this might sound silly but what if the pills upset her tummy... a potentially messy situation...? this might be silly, but it occured to me. Obviously anal is possible on pills generally.