How long does it take for seizure medicines to reach the bloodstream?
- A dose of medication will reach a peak, or maximum, level in the blood 30 minutes to 4 or 6 hours after it is taken. The peak time varies for different drugs.
- The time between taking the medication and reaching the peak level depends on the specific medicine and what form it is in, for example a liquid, tablets, capsule or slow-release form.
- In general, liquids are absorbed quickly.
- Medications in slow-release forms are absorbed gradually over hours and produce the steadiest levels in the blood. These slow-release forms can be taken less often, once or twice a day.
- Eating before a dose may also affect how long it takes to reach a peak level.
I have heard the term "half-life" used to talk about medications. What does this mean?
- A medicine's half-life is the time it takes 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 gotten rid of from the body.
- How often a medicine should be taken depends on the drug’s half-life.
What's the difference between a medicine that has a short half-life and one with a long half-life?
- If a medicine has a short half-life, the amount in your blood may quickly become too low to control your seizures. 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 stay more stable.
- Medicines with long half-lives have more stable levels over a number of hours. This let’s the medicine be taken less often.
When are seizure medicines most likely to work?
Seizure medicines work best when the amount in your system reaches a steady state or constant amount.
- To keep this steady or constant amount, the medicine needs to be taken on a regular basis. The amount you take in needs to equal the amount your body gets rid of.
- Just because a medicine has reached a steady state doesn’t mean that it will control seizures. Only by taking the medicine for a while (usually a number of months) can you tell if the drug is going to work for you.
Is it normal for blood levels of seizure medicines to fluctuate?
- Yes, even when someone has been taking the same medication for a long time the amount in your body will change over the course of the day.
- Missed doses can also be a reason for why medicine levels change.
- 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.
Reviewed by: Joseph I. Sirven, MD | Patricia O. Shafer, RN, MN on 3/2014