Nervous System Home > Phenytoin Dosing

Phenytoin dosing guidelines vary from one product to the next, since the various forms of phenytoin are not interchangeable. The recommended starting phenytoin dosage for extended-release capsules is 100 mg three times daily or 300 mg once daily. With the chewable tablets, the usual starting dose is 100 mg three times a day. With the oral suspension form of phenytoin, dosing typically starts at 125 mg three times per day.

An Introduction to Phenytoin Dosage

The dose of phenytoin (Dilantin®) that your healthcare provider recommends will vary depending on a number of factors, including:
  • The particular phenytoin product
  • Your age and weight
  • The amount of phenytoin in your blood (measured using a blood test)
  • Other medications you may be taking
  • Other medical conditions you may have.
As always, do not adjust your phenytoin dosage unless your healthcare provider specifically instructs you to do so.
It is very important to understand that the different phenytoin products are not interchangeable. Phenytoin capsules (Dilantin Kapseals®) contain phenytoin sodium, while phenytoin tablets (Dilantin Infatabs®) and oral suspension (liquid) contain phenytoin. These are simply different forms of the same chemical. They work the same way in the body, but the doses are not equivalent. In other words, 100 mg of phenytoin in capsule form is not the same as 100 mg of phenytoin in tablet or suspension form.
Written by/reviewed by:
Last reviewed by: Kristi Monson, PharmD;
Last updated/reviewed:
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