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.

Among stoners "THC" is often called the "Total Headache Cure". It's not really, but it will help with some symptoms of tension/stress headaches (reduces tension, duh.) and with migraines it would reduce the nausea.

edited to add:

"often" might be an overstatement.