Onions do more than make me cry, but does that mean I have an allergy or sensitivity to them? The raw, uncooked, or undercooked form of onions gives me severe stomach cramps and diarrhea.  

Apparently,  it is the protein and sulfur present in onions that I react to.  In the raw form, the protein and sulfur are intact so bother my digestive system.  When onions are fried or roasted, the protein is denatured making them tolerable to my very sensitive gut.

A sensitivity to onions is much more common than a true allergy to onions.  If you have a strange feeling (light-headedness, headache, itchy throat) and hives after consuming onions, you may have a rare allergy.  This should be confirmed by a blood test.  Consuming only cooked onions may help, but avoiding them altogether is probably best if you suspect or know you are allergic to onions.  Avoid all items in the onion family as well, including garlic, leeks, shallots, etc.

Lucky for me, I do not experience any of the allergy-related symptoms mentioned above, just painful stomach cramps, indigestions, diarrhea, bloating, etc.  If I consume only well-cooked onions and garlic I experience no discomfort at all.

My diagnosis? Do I have an allergy or sensitivity to onions? Looks to me like just a sensitivity. Phew, because I do like nicely browned, almost caramelized onions in many recipes.