Did a Google on it.
There are potential health risks that must be taken into careful consideration before engaging in fisting. While fisting done with proper care carries a low risk of injury, when done improperly it can result in serious injuries, including ruptured bowels, internal tears, rectal/colonic infection, urinary tract infections, pelvic inflammatory disease, bruising of the cervix, mucosal laceration, muscle tearing, temporary fecal incontinence, or sterility (Inciardi 2000).
Another One
Despite the horror stories that regularly circulate in certain quarters, if fisting is practised in moderation, with patience and care and with due attention to the fistee's feelings it is highly unlikely to lead to serious problems. Trauma and infections in the gut are possibilities but should never happen in responsible fisting. If you think you do have a problem, consult a doctor or a special clinic, and be honest about what you think the cause might be.
Also bear in mind that in engineering terms, fisting exceeds the design specifications of the anus and rectum. If you're fisted frequently there will probably be some long term effects. Certainly being fisted can increase your tendency to fart. You're faeces may increase in girth and you may find you want to shit at irregular intervals. But there should be little long-term effect for those who practise this intense and satisfying form of play as an occasional treat.
My Conclusion:
If done correctly and not often, there is no health risk.