Onion shares the same contact allergen (diallyl disulfide) as garlic because they are in the same botanical family (Allium). Raw onion is more likely to cause problems than cooked because one of its allergenic proteins is destroyed by heat.

However, onion also contains a heat-stable protein (LTP), which means some people react even to well-cooked onion — anaphylaxis to cooked onion has been documented, confirming this. The eye-watering effect of cutting onions is caused by a different chemical (lachrymatory factor), which is an irritant rather than an allergen. Onion has a paradoxical element: it also contains quercetin, a natural antihistamine compound, meaning the same food contains both a potential trigger and a potential anti-inflammatory agent.