This is just a general guideline. Always follow your doctor's and pharmacist's instructions. Most people who suffer from frequent migraines become aware of the onset of the attack 20 - 30 minutes ahead, either from an aura or the first sense of pain or disturbances in smell or vision. It would be best if you take your recommended dose of Imitrex as soon as you become aware that the migraine is imminent. If you can, you should lie down or sit comfortably in a quiet and darkened room.