How common is gangrene as a side effect of colitis?

It Shouldn't be: Gangrene, essentially death of the colon should almost never be seen in this day and age if you are being followed by a GI or colorectal surgeon. It is a very late stage of ischemia and infection and most people will present to the hospital or their doctors long before it gets that bad. The exception would be if you take large amounts of steroids which may mask the signs and symptoms.

Very rare!: Pyoderma gangrenosa is a common skin condition in colitis but gangrene is very rare and is usually related to factors regarding rx than the disease!

Two gangrenes: There might be two totally different things with the name gangrene, one is intestinal gangrene which rarely occurs with proper medical monitoring the other is a difficult skin condition called pyoderma gangrenosum.