How Long Until Seizure Medicine Works?


  • 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.
  • 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.

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.
  • 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.

Authored By:

Joseph I. Sirven MD / Patricia O. Shafer RN MN
Steven C. Schachter, MD

on Tuesday, July 30, 2013

Reviewed By:

Joseph I. Sirven MD / Patricia O. Shafer RN MN

on Wednesday, March 19, 2014


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