Topamax and phone call

I'm 24 and I have had increasingly more migraines as I get older. I now have them 2-3 times a month, and they sometimes last 3-5 days. I went to the doctor, for the first time for migraines, three months ago and he just gave me a to take as needed. those were gone quickly. I had another appointment a few days ago and asked to be put on a daily preventative med. he prescribed topamax. what concerns me is the dosage. he gave me 200 mg to take twice a day. I have never taken any daily med for migraines before, and I see that most people start at 25 mg a day and gradually increase the dosage. I'm very sensitive to medication and I'm worried I can't handle 400 mg of Topamax right off the bat. I was considering cutting the tablets into halves or quarters and starting myself on a lower dosage. perhaps I should just call the doctor for clarification. is this dosage too high for a first time taking Topamax?

any advice much appreciated,

Sarah