Pretty self explanatory - Sam and David have to routinely hound Jason to make sure he's topped up on a substance called Osmium - the only known way they have to control his psychic abilities. It's a bit of a catch-22 though, as he's slowly gaining a tolerance to Osmium - and when he runs out and loses control, it's a pretty bad time for everyone involved. It's all just more motivation for them to find a way to fix his little issue, before it's too late.