Hello. Dilantin (Phenytoin) dosage for maintainence levels for seizure disorder are 300-400mg/ day for adults. However, for some reason this dose is too high for your husband as evidenced by the total phenytoin levels in the blood. You are right about the normal levels - it should be ideally around 10-20mcg/ml. I can think of 2 possible reasons for this. 1. High dosage if body weight is taken into account - The normal adult dosage is for a standard adult of weight 60-65 kg. The dosage according to body weight is 5mg per kg per day. If for any reason your husband s weight is less than 60kg, then a dosage of less than 300mg may be required for him based on calculation. 2. He is on any other medication which is increasing his serum phenytoin levels - Phenytoin is a medicine which interacts with a lot of other medications. You can find the complete list on the internet. Some common ones I have seen in clinical practice are - antifungals like ketoconazole, voriconazole; hormonal therapy like estrogen; anti-arrthmic medicines like amiodarone and other anti-epileptic medicines like diazepam, ethosuximide. Alcohol intake also has a relation to the levels - both acute intake as well as medicines taken for withdrawal can increase the levels. If he is indeed taking any such medications, you can review his medicine list with his physician for an safer alternative. Hope this helps. Regards, Dr Geeta