What could cause sudden sensations of tastes and smells that are not there?

Tasting and smelling things that are "not there" can be caused by many things...a few are: fungal infections in the sinus cavities and some medications. Metallic tastes and smells can be from medications. It can make you feel insane to taste and smell things that you know you have not tasted/eaten. If you are noticing this all the time or if it comes and goes, it is probably best to go to an ENT...ear, nose and throat physician. Some epileptics note a different taste or smell right before they have a seizure..a form of aura.