Onions have a universal appeal. They are safely consumed by most people. However, consuming large quantities of onions can lead to stomach distress and gastrointestinal irritation that may result in nausea and diarrhea. The bulbs contain a considerable proportion of alimentary matter, principally mucilage, particularly when boiled, but in dyspeptic habits they may cause flatulence, thirst, and headache. There are no known interactions with drugs except that they can potentiate the action of coagulants.³⁴

 

As onions are sliced, cells are broken, allowing enzymes called alliinases to break down sulfides and generate sulfenic acids (amino acid sulfoxides). Sulphenic acids are unstable and decompose into a volatile gas called syn-propanethial-S-oxide. The gas dissipates through the air and eventually reaches the eye, where it reacts with the water to form a dilute solution of sulfuric acid. This acid irritates the nerve endings in the eye, making them sting. Tear glands produce tears to dilute and flush out the irritant. Supplying ample water to the reaction prevents the gas from reaching the eyes. Eye irritation can, therefore, be avoided by cutting onions under running water or submerged in a basin of water. Rinsing the onion and leaving it wet while chopping may also be effective. Chilling or freezing onions prevents the enzymes from activating, limiting the amount of gas generated. Using a sharp blade to chop onions will limit the cell damage and the release of enzymes that drive the irritation response. The volume of sulphenic acids released, and the irritation effect, differs among Allium species.³⁵  Since onions can induce perspiration some herbalists advise to use caution or avoid when there is profuse sweating.³⁶