All of us who have cut an onion know what effect the onion has on us: bland (non-irritating) tearing of the eyes, with runny nose (burning and acrid). the running of the nose peels away (excoriates the skin under the nose where the watery discharge occurs. At the same time there is severe sneezing.      

Allium cepa is most often used for an acute attack of hayfever. Note that is has exactly the opposite symptoms of Euphrasia, another acute hayfever remedy. Any allergy atack which has the above symptoms (sneezing, nose running and chapping upper lip) will respond well to this remedy.     

A little known use is for the treatment of acute pain in an amputated limb
(neuralgia of the phantom limb): the patient comlains of pain in the missing limb, hence the name "phantom".