Find Out the Cause

Is there ever a reason, even for health reasons, to leave a vegetarian diet to go back to eating animal products?

There are plenty of reasons for it if you are keen on eating animal materials. If you fall sick, it is not because you are eating a vegetarian diet.

Disease means that you have destroyed your ease from a natural condition. When you destroy your ease, you get disease. So find out the cause that disturbed your ease, and correct it.