Re: what are the medical reasons you can't box ?
As a serious answer, any sort of heart or brain condition is obviously going to prevent you from boxing.
It may also vary from country to country. I remember reading something about diabetics being banned in England (but they were reviewing the rule and maybe they aren't anymore).
Is there some specific condition you have that you are wondering about? Best way is to get a medical form from your trainer and take it to your doctor.
Severe diarrhea is a no go.