Let us say that someone has Celiac and can't find gluten free matzos - should he eat regular matzos and risk illness?
Ra Moshe Feinstein says that since the limit of obligation to fulfill מצוות עשה, is one fifth of your money, any mitzva that causes pain that one would give more than a fifth of his money to avoid is not obligatory. So if he would get so sick that he would be willing to pay more than a fifth of his money to avoid the suffering he is not obligated.
If he would experience discomfort but no big deal then apparently he is obligated.
If he is ENDANGERED by doing mitzva then apparently he should not do it and if he insists on doing the mitzva and makes a bracha, it is a bracha li-vatala [Rav Moshe Shternbuch]. Some argue that it is not a bracha livatala because while not obligated in the mitzva, if he fulfills it then he is a tzadik.
I would argue that if he fulfills it then he is being foolish. The Torah says "ונשמרתם מאד לנפשותיכם" teaching us that it is the will of our Creator that we guard our health. If one fulfills a mitzva that he is not obligated to fulfill and thereby endangers himself, that is not the will of his Creator. So a person with Celiac should not [according to this logic] eat regular matzos and if he does then he shouldn't make a bracha.
This is a big topic that I just touched slightly to get people thinking.
