When I have a headache, I use aspirin or some type of Indica blend with water based on where my tolerance is at and I don't recall the last time a headaches persisted for longer than an hour.

That being said, mind altering substances are always a risk. All the scientific studies i rely on source from erowid.org or wikipedia and heavily support the theory that cannabis is safe in vapor and edible form and is one of the best general analgesics.