Q: I am currently taking my 600 mg/day of Dilantin in 3 divided doses. I have recently added lamictal to try to help breakthrough seizures and am up to 300 mg/day with a target of 400 mg/day if no serious side effects. My question: Is there any benefit to divided doses of the Lamictal, or is it just as effective in one daily dose?
A: In drug therapy, we describe how long a drug lasts in the body by the term "half-life." This is the length of time it takes for the body to get rid of half of the amount of medication in the body. The longer the half-life, the longer the medication stays in the body. Presumably, drugs with long half-lives do not need to be given as often. By tradition, physicians give medications in interval of the drugs half-life, but this means that blood levels will drop by half before the next dose. In many situtations, this amount fluctuation is fine, in other cases it is not. Giving one lump dose for a drug with a short half life would cause huge drops in levels between doses. On the other hand, dividing doses lowers peaks levels (thus reducing the potential for side effects) and raise trough levels (thus reducing the risk of seizure breakthrough). However, having too many dose times is inconvient and could lead to missing medications.
The half life of lamotrigine (Lamictal) is estimated to be about 15 hours in humans, but may be less if given with a drug that increases liver metabolism, such as phenytoin (Dilantin) or carbamazepine (Tegretol). This is why it is usually given twice-a-day.
In some persons a large fluctuation in blood levels over the day may not make much of a difference. However, I presume that you are starting lamotrigine because your seizures are not well controlled. therefore you probably would not want large fluctuations in your blood levels and should stay with divided doses.