Leftover Onions can be poisonous

Philippines
@rosey19 (954)
November 11, 2009 2:40am CST
Do believe that leftover onions are not safe to use even if they are place inside the refrigerator? There was an email circulating now giving warning to everybody not to use leftover onions for cooking because it can be poisonous. It was a chemist who said it, since bacteria can easily penetrate to it when it was already slice. Do you have experiences having been poison or having stomach trouble after eating this kind of spice?