Ok, I'll say it... Marijuana (sativa, not indica). That prescription written, let me warn you it is a motivation killer if used excessively! By excessively I mean more than a quarter gram a day. If you use enough to "get stoned", you're screwed. Cook a pizza and kiss that day goodbye! When you reach an ounce a month like I did, it takes at least a month of staying off it to get back to normal (which is why I quit completely). If one has the self control to not over do it, its a miracle plant. Treat it with respect and it will serve you well.
The number one way to reduce stress is to identify the triggers and work to avoid them. Admittedly, its not that simple in all cases ~ I know.