The following supplements have been found helpful for migraines:
Omega-3 fatty acids: Including fish, flax seeds, walnuts. Two to 6g/day has been show to have significant effects(1) perhaps by reducing inflammation, platelet inhibition or vasorelaxant effects. Fish oil is usually very well tolerated. Quality product is essential (5).
Vitamin B2 (Riboflavin): Studies show significant preventive effects at 200mg twice daily(6,7). Side effects are rare.
Magnesium: Can be effective in prevention(8). Magnesium treatment may reduce in cortical neuronal excitability or may alter magnesium-dependent circadian regulatory mechanisms that are often disturbed in migraine(9). 400-1000mg orally as chelated magnesium or magnesium glycinate (other forms may not be absorbed as well). Can cause diarrhea, but usually well tolerated.
5-HTP: 50mg 1-3 times daily. May cause mild gastrointestinal disturbances including nausea, heartburn, flatulence, and feelings of fullness. There have been rare reports of eosinophilic myalgia syndrome (6).
CoQ10: 150-300mg daily gave 50% reduction in migraines(10,11). Very well tolerated.
Melatonin: 1-10mg daily (or as needed) before bed especially with sleep disorders(12). Recommend starting with 1-3 mg and up-titrating as needed. Side effects are rare and include vivid dreams or nightmares, stomach cramps, dizziness, headache, irritability, decreased libido, male gynecomastia, and decreased sperm count. Individuals experiencing morning drowsiness after taking melatonin at night should take less of the supplement.