Sumatriptan can cause drowsiness, it varies person to person how much but once my dad has taken his he cannot drive for the rest of the day. He has other conditions that make him sleepy however so its probably a combined effect for him.
There are many treatments for migraines however so don't panic, if this one turns out to not be the right one for you, there are more! I hope the drowsiness reduces, but in a way, its good for migraine recovery to sleep it off. I find unless I sleep I don't fully recover and can develop a second migraine sooner.
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