I had botox in my anus last week to help anal fissure heal. It was done under a general anesthetic (with a curette of the fissure at the same time). The botox didn't hurt at all (although the curette did for a few days).
Since the botox, I find that when I cough or sneeze I have mild bowel incontinence. This is probably also connected to the fibre and stool softeners I take every day to help with the anal fissures. I also find that when I need to have a BM, I can't hold it in very long, and over the past week, I have messed myself several times when the poop just starts oozing out (as it is quite soft). I am finding the diapers a godsend for this, although I do have to change more than usual.