Imitrex is one of those drugs I've never tried, yet I know plenty of migraine sufferers that say it works once, twice, then it's a waste of time...No experience with Topamax. However, my wife takes a migraine drug called Amerge. They work like magic, but they are expensive. Our co-pay is $18 a pill.
There is another one called Imitrex that is like Amerge. They work by constricting vessels in your brain, and they can seriously raise blood pressure as a side effect. My wife has never had any bad experience though, and I think the bp thing is fairly rare. For her, no detectable side effects at all. These drugs are non-narcotic (they are not "pain killers").
I looked up Topamax and it is primarily considered an anti-seizure drug (anti-epileptic drug). This type of drug usually has unpleasant side effects.
Never heard of Amerge; I'll go look it up...I've tried several migraine meds that you take at the onset of a migraine; since none of them work long-term, I'm leaning toward trying the preventative instead. My doc offered it to me last year and I opted out, figured I'd just hope for the best with the meds I was on, but it's clear now that none of them are really doing the job...
And yes, Topamax was originally deemed a drug for epilepsy, but it's used for other conditions as well...my best friend takes it daily for issues unrelated to both epilespy or migraines...that part does kind of worry me though...