Back in the mid-1990s there was a TV show called The Invisible Man. The main character ended up getting an experimental drug called Quicksilver, that would allow him to go invisible for a period of time. If he didn't get a recharge, his eyes would turn red and he would go mad.
The best part of the series is when he found out that the serum that made him go invisible was actually from a big foot. So, the reason that nobody could find any big foots is that they were invisible. And if they stayed invisible too long, they would stay that way, go mad and die.
I actually love that! Sounds like a very well thought through plotline! I'll have to check that out sounds very interesting