The more you practice magic, the more you have to use it. So...there's somebody who has to use magic. Normally it's not a problem; they just use it for little things throughout the day, most of them not anything that would be noticed even by somebody looking right at them. But then there's something tha prevents them from doing magic: a curse, or they're trapped in a warded house, or something. They're not alone. And because they can't use magic, it builds up in them. The first side effects are small, easily ignored, usually a sign to use magic. Later, they start acting like they're on drugs. If it goes on long enough they'll die.