I have used cannabis to treat migraines for about the past 6 years. I have found it to be the best medicine for migraines compared to other things that I have tried: namely, over-the-counter analgesics (such as Tylenol, ibuprofen, aspirin, Alleve, etc.) and to the prescription drug Midrin.
With smoked cannabis, the relief is almost immediate (5 -15 minutes), whereas with the oral medications relief takes 30-120 minutes. Also, the effectiveness of pain and nausea cessation seems to be better with cannabis. I can usually kill a migraine with a few puffs of cannabis, whereas I usually never get COMPLETE relief with the oral medications until I've slept for a few hours.
As you probably know already, a migraine can literally consume a whole day to a week of your life (and then some) if it is bad enough. I discovered that cannabis usually allows me to overcome the migraine and get on with my life almost immediately. If you work or have a busy lifestyle, this is something else to consider.
Another pitfall with some of the oral prescription medications, and one that I experienced with Midrin, is that they can actually cause you to have more migraines more often!!!!! This negative side effect of these drugs is a physiological response similar to an addiction. Your body decides that it wants more of the drug, so it intentionally creates a migraine to force you into taking the medication! When I first started taking Midrin back in the early 90's, I would have (on average) about 1 migraine every 2 weeks. After being on Midrin for about 4 months, I was having migraines about every other day! It was then that my doctor explained this weird side effect to me, and I stopped taking Midrin. Of course, not all people experience this side effect but many do.
There are also other lifestyle changes that you may want to consider to reduce incidence of migraines. For me, I found that treating my high blood pressure helped reduce the number of migraines, as did eliminating the artificial sweetener Nutrasweet from my diet.