A dose of medication will reach a peak, or maximum, level in the blood 30 minutes to 6 hours after it is taken. The time between taking the medication and reaching the peak level depends on the specific medicine, its form (liquid, tablet, capsule, slow-release form) and, in some cases, the food consumed before taking it. In general, liquids are most rapidly absorbed. Medications in slow-release forms are absorbed gradually and produce the steadiest levels in the blood, so doses can be taken fewer times per day.
A medicine's half-life is the time required for its level in the blood to be reduced by one-half. The half-life of each seizure medicine depends on how quickly it is broken down and eliminated from the body. A medicine's half-life determines how often it should be taken.
If a medicine has a short half-life, the amount in your blood may quickly become too low to control your seizures, so you will need to take another dose. Medicines with short half-lives ideally should be taken several times a day. Then you can take a smaller amount each time and your blood level will not become too high, which may produce unwanted side effects. The shorter time between doses will prevent the level from becoming too low, which may allow seizures to occur. Medicines with long half-lives, on the other hand, have more stable blood levels and can be taken less frequently.
Seizure medicines are fully effective when their blood levels have reached a steady state, or equilibrium. Someone who reaches a steady state is taking a constant amount of medication and the level in the blood is remaining fairly constant. In other words, the amount of medication the person takes and the amount that is metabolized and excreted from the body (in urine, for instance) are equal. This usually takes an amount of time equivalent to five half-lives.
Yes, even when someone has been taking the same medication for a long time and the blood levels are in steady state, the levels will fluctuate over the course of the day. Dose-related side effects are more likely to occur at times of the highest (peak) levels and a seizure is more likely to occur at times of the lowest (trough) levels.
Topic Editor: Steven C. Schachter, M.D.
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