What Causes Poor Coordination And Vision While On Lamictal?

My daughter is taking Lamictal for bipolar disorder. She was given an increase in her medication from 300 mg to 400 mg per day 2 weeks ago. She realized last night that her new prescription was for 200 mg tablets instead of the previous 100 mg tablets, which has resulted in her taking 800 mg per day (2 tablets am/ 2 tablets pm) instead of 400 mg. She has experienced terrible coordination issues and vision issues. How does she go about reducing the medication amounts to her proper dosage without triggering an adverse reaction from making to sudden of a change?