This article describes phenytoin injection’s dosages and strength and gives details on how it’s given. If you want to know more about phenytoin injection, see this overview article. It covers details about the drug’s uses and side effects, ways to save on cost, and more.
Phenytoin injection is a generic medication. It’s not available in a brand-name version. Phenytoin also comes in forms that are taken by mouth, but those forms aren’t described in this article. To learn more about those forms, talk with your doctor.
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Boxed warning: Risk of cardiovascular problems with rapid infusion
Phenytoin injection has a boxed warning for the risk of cardiovascular problems if it’s infused too quickly. A boxed warning is the most serious warning from the Food and Drug Administration (FDA).
For details about this boxed warning, see the “Dosage of phenytoin” section below. See the end of this article for more information about this warning.
Form and strength of phenytoin
Phenytoin injection comes as follows:
- Form: injection administered by a healthcare professional, usually given intravenously, which means into a vein
- Strength: 50 milligrams per milliliter (mg/mL)
Dosage of phenytoin
This article describes the recommended dosages for phenytoin injection. The drugmaker provides these dosages.
Phenytoin injection is given by a doctor or other healthcare professional. Your doctor will determine the dosage that best meets your needs.
Usual recommended dosages of phenytoin in adults
Phenytoin injection can be given into a vein (intravenously) as either a loading dose or an infusion. A loading dose is a higher dose of medication given at the beginning of treatment so the drug can start working more quickly. An infusion is an injection given into your vein over time.
Due to the boxed warning for this medication, the rate of phenytoin injection should not be faster than 50 mg per minute in adults. In nonemergency situations, phenytoin injection should be given more slowly, either as a loading dose or intermittent infusion. See the end of this article for more information about the boxed warning.
Because phenytoin has a risk of cardiovascular and other problems when given via injection, oral phenytoin is preferred when possible. If your doctor starts you on phenytoin injection, they may switch you to a form that’s taken by mouth. But if needed, phenytoin injection may be used in the short term instead of the oral form.
The recommended loading dosage and maintenance dosage of phenytoin injection for adults are outlined below. The dosage is shown in milligrams (mg) per kilogram (kg) of body weight. For reference, 1 kg is about 2.2 pounds (lb).
| Purpose | Loading dosage | Maintenance dosage |
|---|---|---|
| treatment of status epilepticus (prolonged, severe seizures) | 10–15 mg/kg | 100 mg given by mouth or IV every 6–8 hours |
| prevention and treatment of seizures during brain surgery or nonemergency situations | 10–15 mg/kg | 100 mg given by mouth or IV every 6–8 hours |
| treatment of tonic-clonic (grand mal) and psychomotor (temporal lobe) seizures | 10–15 mg/kg | 100 mg given by mouth or IV every 6–8 hours |
Dosages of phenytoin in children
As with the dosage for adults, when given to children, phenytoin injection can be given into a vein (intravenously) as either a loading dose or an infusion.
Due to the boxed warning for this drug, the rate of phenytoin injection in children should not be faster than 1–3 mg per kilogram per minute or 50 mg per minute, whichever is slower. See the end of this article for more information about the boxed warning.
In nonemergency situations, phenytoin injection should be given more slowly, either as a loading dose or intermittent infusion.
The recommended loading dosage and maintenance dosage of phenytoin injection for children are outlined below.
| Purpose | Loading dosage | Maintenance dosage |
|---|---|---|
| treatment of status epilepticus (prolonged, severe seizures) | 15–20 mg/kg | 100 mg given by mouth or IV every 6–8 hours |
| prevention and treatment of seizures during brain surgery or nonemergency situations | 15–20 mg/kg | 100 mg given by mouth or IV every 6–8 hours |
| treatment of tonic-clonic (grand mal) and psychomotor (temporal lobe) seizures | 15–20 mg/kg | 100 mg given by mouth or IV every 6–8 hours |
Dosage adjustments for phenytoin
Your doctor will prescribe a dosage of phenytoin injection based on several factors. These include:
- the specific condition being treated and how severe it is
- your age
- other health conditions you may have
- whether you have kidney or liver problems
Frequently asked questions
Below are answers to some commonly asked questions about phenytoin injection and its dosage.
Do older adults need a lower dose of phenytoin?
Maybe. Some older adults may need a lower dose of phenytoin injection. (In general, the term “older adults” refers to people ages 65 years and older.)
Older adults may clear phenytoin from their bodies more slowly, which could cause phenytoin to reach higher levels in the body. This may increase the risk of serious side effects. That’s why older adults may need lower doses of phenytoin injection. Or the drug may need to be given less frequently.
Talk with your doctor if you have questions about what dose of phenytoin is right for you based on your age.
Is phenytoin used long term?
No, doctors don’t typically prescribe phenytoin injection as a long-term treatment. Instead, it’s usually recommended for short-term use.
If you have questions about how long you’ll need to be given phenytoin injection, talk with your doctor.
How phenytoin is used
A healthcare professional will give you phenytoin injection. Your dosage will depend on several factors, including why it’s being prescribed and your age.
Phenytoin injection can be given into a vein (intravenously) as either a loading dose or an infusion. A loading dose is a higher dose of medication given at the beginning of treatment so the drug can start working more quickly. An infusion is an injection given into your vein over time.
Phenytoin injection is also approved to be given into a muscle (intramuscularly). But getting it this way can increase the risk of serious reactions where it’s injected. It may also be poorly absorbed. For these reasons, it’s usually best to avoid injecting phenytoin into a muscle. If phenytoin injection does need to be given into a muscle, higher doses may be needed.
Phenytoin coupon
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Get free cardWhat to ask your doctor
This article describes the usual recommended dosages for phenytoin injection. If your doctor prescribes this medication, they’ll determine the dosage that’s best for you.
Let your doctor know if you have concerns or questions about your treatment plan.
Here’s a list of some possible questions you may want to ask your doctor:
- What factors will determine my dosage of phenytoin injection??
- Is the dosage of phenytoin injection the same as with the oral forms?
- How long will I need to be given phenytoin injection?
Boxed warning for phenytoin
Phenytoin injection has a boxed warning for the risk of cardiovascular problems if it’s infused too quickly. A boxed warning is the most serious warning from the Food and Drug Administration (FDA).
It’s very important that phenytoin injection is given at the right rate. If it’s given too quickly, it can cause severely low blood pressure and irregular heart rhythms. In some cases, these can lead to heart attack or death.
The risk of these serious side effects may be higher in:
- older adults
- people who have low blood pressure
- people in critical condition
But these serious side effects are also possible in people who don’t have these risks. These side effects can also occur when phenytoin injection is given at or below the right rate.
Due to the risk of serious side effects, phenytoin injection should not be given more quickly than the following rates:
- Adults: 50 mg per minute
- Children: 1–3 mg/kg/min or 50 mg/mi, whichever is slower
In nonemergency situations, phenytoin injection should be given more slowly, either as a loading dose or intermittent infusion.
Your doctor will monitor your heart closely while giving phenytoin injection and afterward. If you have serious side effects with phenytoin injection, your doctor may prescribe an even slower rate, or they may stop the drug.
Because phenytoin injection has a risk of cardiovascular problems, oral phenytoin is preferred when possible. If your doctor starts you on phenytoin injection, they may switch you to a form that’s taken by mouth. But if needed, phenytoin injection may be used in the short term instead of the oral form.
For details, see the “Dosage of phenytoin” section above. You can also talk with your doctor to learn more.
Disclaimer: Optum Perks has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.