I agree with Fluffy on this one. The Dr.'s and Neurologists told us the same thing when my husband was on dilantin. We were using the generic (no insurance) and his dilantin levels were fluctuating so much from month to month. They would keep increasing his dilantin every month. They said it varies so much with the generic because sometimes they have more "filler" in the capsule than others. They may have 80% filler one month and 30% filler the next. Its just not consistant enough.
You should also make sure that you have your liver tested every 3 months because the dilantin does cause liver damage.
Have a great day.