Finding the Best Dosage of Medication


Every person is different and the dose of medicine they need will depend on many factors. The ‘best dose’ is the one that controls seizures with the least amount of side effects. When a drug is tested, the Food and Drug Administration (FDA) approves a dosage range, or how much a person should be given of each drug. Yet, this is just a guide that the doctor uses to find the best dose for each person. Some things that affect the ‘best dose’:

  • The individual medicine and how it works: Some medicines need to be given in higher doses than others. Some work best at low doses.
  • A person’s age and weight: Older people often need lower doses than younger adults. Children’s bodies break down medicines differently too, depending on their age.
  • Length of time taking the medicine: A person’s body gets used to a medicine over time. Some medicines build up quickly to a good level, others need to be adjusted slowly.
  • Other medicines taken: some medicines may affect how a drug is absorbed or broken down in the body and the amount that gets to the brain.
  • Other side effects or health problems: Some medicines could make side effects or other problems worse, for example blurry vision, coordination or unsteady walking. Other medicines may help some issues, for example many seizure medicines help mood and pain.

To find the best dose of a medicine, you and your doctor or team may want to know:

  • How often you are having seizures and if they have changed since the dose was changed.
  • If you are having side effects that are bothering you,
  • If a higher or lower dose would work better for you.
  • If you have been taking the medicine long enough to judge whether or not it’s been helpful.
  • How much of the medicine is getting into your body and the highest dose you can tolerate.

Blood tests can help you learn how your body is tolerating the medicine and how much medicine is in your bloodstream. This is a guide to how much medicine is getting to your brain. The blood test can tell if you are getting ‘enough’ medicine to likely control seizures, or if you are getting ‘too much’ that may be causing side effects.

Blood tests can also measure whether the drug has reached a ‘steady state’ in the body. Each seizure medicine has a ‘half-life’, which is the amount of time it takes for the body to get rid of half of the medicine.

Why is this important?

  • It takes the body 5 times the half-life a drug to reach a steady amount in the body. So if a half-life of a medicine is one day, then it will take at least 5 days to reach a steady amount. Some medicines with long half-lives, don’t reach a steady amount for up to three weeks!
  • If a blood test is drawn too soon after starting a medicine, it won’t tell you if the drug has reached a steady state. You need to wait long enough for the drug to reach a steady state before checking the blood level.
  • If a person’s been on a consistent dose for long enough, the blood test can be used to see if a person’s been taking the medicine regularly.

Other tips about blood levels:

  • For some medicines, blood levels are used frequently. For example blood levels of many of the older medicines (such as phenobarbital, phenytoin (Dilantin), carbamazepine (Tegretol, Carbatrol), or valproic acid/valproate (Depakene, Depakote) have well-known ranges. When used correctly, the levels can help you and your doctor know how you are tolerating the medicine. These blood tests are especially helpful if someone is taking other medicines that may change the amount of drug in your body.
  • Blood level tests are also available for many of the newer medicines, but it’s not as easy to know what they mean. Some people do well at high levels of medicines, and others don’t.
  • Blood tests for some of the newer seizures medicines aren’t very helpful, because the blood test can’t measure what the drug does in the brain. Or the medicine breaks down into different byproducts and the blood test doesn’t measure all of these. 
  • The time the blood test is drawn is very important.
    • Usually it’s best to check a blood level before you are due to take a dose, or first thing in the morning. This will measure the lowest amount of medicine in your body.
    • Sometimes a blood test is done to check the highest amount of medicine in your body to see if this relates to side effects you may have. In this case, it’s best to have the blood test done when the medicine peaks in the bloodstream, usually about 1 to 4 hours after the highest dose of the day. Or your doctor may ask that the blood level be done when you are feeling side effects.

If you have had only one or two seizures, you are likely to get good seizure control from low doses of a seizure medicine. Others may need higher amounts.

  • Even if you have low levels in your blood, if you are doing well, then the medicine dose may be left alone for a while.
  • For some medicines, the dose may be increased by small amounts very slowly to help your body get used to the medicine. The amount in the blood or how often you are having seizures may not be as important as how fast or slow the dose is increased.
  • As your body gets used to a medicine, your body may break it down faster and the level may drop. So to prevent more seizures, the dose may need to be raised until it reaches a steady amount in your body.

Two factors should guide changes in medicine doses: side effects and efficacy or how well the medicine controls seizures. Efficacy is often impossible to predict accurately. Usually the dose of a medicine is raised until you reach the lowest amount of drug that will control seizures and not cause side effects. Sometimes experience helps your doctor know how much to give you and when to change the dose. Other times, there’s a bit of a ‘trial and error’ approach to find out what is best for each person. The following guidelines are helpful in adjusting the dosage of seizure medicines:

  • If a person is having breakthrough seizures without missing any doses, it may be time to change the dose. Talk to your doctor and show them your seizure records.
  • Sometimes spreading out how many times a day you take a medicine can lessen side effects. However, the more often you need to take pills, the more likely you are to forget a dose.
  • Sometimes changing the dose make it easier to take the medicine. For example, raising the dose may help it last longer over the course of a day. Or it may allow you to take the medicine twice a day instead of three.
  • Taking a medicine after food and at bedtime may lessen stomach upset after taking a dose.
  • Taking the largest dose at night may help you tolerate the medicine better. For example, if you get tired after a higher dose, it won’t bother you as much if you are going to sleep anyway. It may also help you feel more alert and mentally sharp during the day.
  • Some medicines are best to take when the person is more likely to have seizures. For example, people who have seizures only at night may be asked to take their seizure medicine at dinner or before bedtime so the peak amount of medicine it working at night.

Sometimes it is hard to know if symptoms are related to brief seizures, mood, or side effects of medicines. Other medical problems may be confused with medicine effects too. Here are a few tips to consider:

  • Side effects related to the dose of medicine often start about 30 minutes to 2 hours after a dose taken and last for more than 15 minutes.
  • Some side effects occur soon after starting a medicine or changing a dose. These symptoms may go away on it’s own after a while.
  • Some symptoms may occur if you take it on an empty stomach. Taking it after food may make it go away.
  • Some may occur if you take doses too close together. Try to keep the medicines spaced out on a regular schedule.
  • Some people may get side effects that aren’t related to time of day or dose. It may be due to the drug itself. Some of these side effects may go away. Others won’t and may mean that a person can’t take the medicine. If problems are bothersome and not going away, it’s important to talk to a doctor.
  • If serious problems occur, such as a rash, fever, or other signs of an allergic reaction, it’s important to get emergency medical attention.  

Authored By:

Joseph I. Sirven MD and 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


Epilepsy Centers

Epilepsy centers provide you with a team of specialists to help you diagnose your epilepsy and explore treatment options.


Epilepsy Medication

Find in-depth information on anti-seizure medications so you know what to ask your doctor.


Epilepsy and Seizures 24/7 Helpline

Call our Epilepsy and Seizures 24/7 Helpline and talk with an epilepsy information specialist or submit a question online.


Tools & Resources

Get information, tips, and more to help you manage your epilepsy.


Find an Epilepsy Specialist

Ready for help? Find an Epilepsy specialist who can help guide you through your epilepsy journey.