My Mom Has Migraine Since Last 18 Years, I Happened

My mom has migraine since last 18 years, I happened with in every 15 to 20 days, We have consulted from st. Stephen hospital, IHBAS hospital and few private neurologist also. Every time when we start treatment for 1 or 2 months, medicine shows positive result and frequency and impact of migraine decrease, but after that that medicine starts negative impact i.e. less sleep, weakness, tightness in hands and legs, unconsciousness, low blood pressure, no relax in mind etc. She took Voltaflam medicine, which is the only medicine by which her migraine can be reduced, she started taking one tablet as prescribed by St. Stephen doctors 15 years ago, now she has to take at least 4 tablet at a time to get relax from migraine. In worst case she has to take 4 tablet + 2 tablet after few hours to get relief. Please suggest what to do? We have tried homeopathic treatment also, but no result.