Know the basics
What is phenytoin used for?
Phenytoin is used to prevent and control seizures (also called an anticonvulsant or antiepileptic drug). It works by reducing the spread of seizure activity in the brain.
OTHER USES: This section contains uses of this drug that are not listed in the approved professional labeling for the drug but that may be prescribed by your health care professional. Use this drug for a condition that is listed in this section only if it has been so prescribed by your health care professional.
This drug may also be used to treat certain types of irregular heartbeats.
How should I take phenytoin?
Read the Medication Guide provided by your pharmacist before you start taking phenytoin and each time you get a refill. If you have any questions, ask your doctor or pharmacist.
The tablets may be chewed thoroughly before swallowing or swallowed whole.
Take this medication by mouth usually 2 or 3 times a day, or as directed by your doctor. This product is not recommended for use once a day. You may take it with food if stomach upset occurs. Take this medication with a full glass (8 ounces or 240 milliliters) of water unless your doctor directs you otherwise.
Use this medication regularly in order to get the most benefit from it. It is important to take all doses on time to keep the amount of medicine in your body at a constant level. Remember to use it at the same times each day. Dosage is based on your medical condition and response to therapy.
Products that contain calcium (e.g., antacids, calcium supplements) and nutritional tube-feeding (enteral) products may decrease the absorption of phenytoin. Do not take these products at the same time as your phenytoin dose. Separate liquid nutritional products at least 1 hour before and 1 hour after your phenytoin dose, or as directed by your doctor.
Do not stop taking this medication without consulting your doctor. Seizures may become worse when the drug is suddenly stopped. Your dose may need to be gradually decreased.
Inform your doctor if your condition does not improve or worsens.
How do I store phenytoin?
Phenytoin is best stored at room temperature away from direct light and moisture. To prevent drug damage, you should not store phenytoin in the bathroom or the freezer. There may be different brands of phenytoin that may have different storage needs. It is important to always check the product package for instructions on storage, or ask your pharmacist. For safety, you should keep all medicines away from children and pets.
You should not flush phenytoin down the toilet or pour them into a drain unless instructed to do so. It is important to properly discard this product when it is expired or no longer needed. Consult your pharmacist for more details about how to safely discard your product.
Know the precautions & warnings
What should I know before using phenytoin?
In deciding to use a medicine, the risks of taking the medicine must be weighed against the good it will do. This is a decision you and your doctor will make. For this medicine, the following should be considered:
Tell your doctor if you have ever had any unusual or allergic reaction to this medicine or any other medicines. Also tell your health care professional if you have any other types of allergies, such as to foods, dyes, preservatives, or animals. For non-prescription products, read the label or package ingredients carefully.
Appropriate studies performed to date have not demonstrated pediatric-specific problems that would limit the usefulness of phenytoin injection in children.
Appropriate studies performed to date have not demonstrated geriatric-specific problems that would limit the usefulness of phenytoin injection in the elderly. However, elderly patients are more likely to have age-related liver, kidney, or heart problems, which may require an adjustment in the dose for patients receiving phenytoin injection.
Is it safe to take phenytoin during pregnancy or breast-feeding?
There are no adequate studies in women for determining risk when using this medication during pregnancy or while breastfeeding. Please always consult with your doctor to weigh the potential benefits and risks before taking this medication.
Know the side effects
What are the side effects of phenytoin?
Get emergency medical help if you have any of these signs of an allergic reaction: hives; difficulty breathing; swelling of your face, lips, tongue, or throat. You may be more likely to have an allergic reaction if you are African-American.
Report any new or worsening symptoms to your doctor, such as: mood or behavior changes, depression, anxiety, or if you feel agitated, hostile, restless, hyperactive (mentally or physically), or have thoughts about suicide or hurting yourself.
Call your doctor at once if you have a serious side effect such as:
- fever, swollen glands, body aches, flu symptoms;
- skin rash, easy bruising or bleeding, severe tingling, numbness, pain, muscle weakness;
- upper stomach pain, loss of appetite, dark urine, jaundice (yellowing of the skin or eyes);
- chest pain, irregular heart rhythm, feeling short of breath;
- confusion, nausea and vomiting, swelling, rapid weight gain, urinating less than usual or not at all;
- new or worsening cough with fever, trouble breathing;
- tremor (uncontrolled shaking), restless muscle movements in your eyes, tongue, jaw, or neck;
- patchy skin color, red spots, or a butterfly shaped skin rash over your cheeks and nose (worsens in sunlight); or
- severe skin reaction — fever, sore throat, swelling in your face or tongue, burning in your eyes, skin pain, followed by a red or purple skin rash that spreads (especially in the face or upper body) and causes blistering and peeling.
Less serious side effects may include:
- slurred speech, loss of balance or coordination;
- swollen or tender gums; or
- headache, dizziness, nervousness, or sleep problems (insomnia).
Not everyone experiences these side effects. There may be some side effects not listed above. If you have any concerns about a side-effect, please consult your doctor or pharmacist.
Know the interactions
What drugs may interact with phenytoin?
Using this medicine with any of the following medicines is not recommended. Your doctor may decide not to treat you with this medication or change some of the other medicines you take.
Using this medicine with any of the following medicines is usually not recommended, but may be required in some cases. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.
- Abiraterone Acetate
- Dabigatran Etexilate
- Doxorubicin Hydrochloride Liposome
- Eslicarbazepine Acetate
- St John’s Wort
- Ulipristal Acetate
- Vincristine Sulfate
- Vincristine Sulfate Liposome
Using this medicine with any of the following medicines may cause an increased risk of certain side effects, but using both drugs may be the best treatment for you. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.
- Estradiol Cypionate
- Estradiol Valerate
- Ethinyl Estradiol
- Ethynodiol Diacetate
- Folic Acid
- Medroxyprogesterone Acetate
- Valproic Acid
Does food or alcohol interact with phenytoin?
Phenytoin may interact with food or alcohol by altering the way the drug works or increase the risk for serious side effects. Please discuss with your doctor or pharmacist any potential food or alcohol interactions before using this drug.
What health conditions may interact with phenytoin?
Phenytoin may interact with your health condition. This interaction may worsen your health condition or alter the way the drug works. It is important to always let your doctor and pharmacist know all the health conditions you currently have, especially:
- Blood or bone marrow problems (eg, agranulocytosis, leukopenia, thrombocytopenia) or
- Diabetes or
- Heart failure or
- Heart rhythm problems or
- Hypotension (low blood pressure) or
- Lymphadenopathy (lymph node problems) or
- Porphyria (an enzyme problem)—Use with caution. May make these conditions worse.
- Heart block (eg, Adams-Stokes syndrome, AV block, or sinoatrial block) or
- Sinus bradycardia (slow heartbeat)—Should not be used in patients with these conditions.
- Hypoalbuminemia (low albumin in the blood) or
- Kidney disease or
- Liver disease—Use with caution. The effects may be increased because of slower removal from the body.
Understand the dosage
The information provided is not a substitute for any medical advice. You should ALWAYS consult with your doctor or pharmacist before using this medication.
What is the dose of phenytoin for an adult?
Usual Adult Dose for Seizures
Oral (except suspension) Loading dose: Only when indicated for inpatients.
1 g orally divided in 3 doses (400 mg, 300 mg, 300 mg) given at 2 hour intervals. Then normal maintenance dosage started 24 hours after loading dose.
Initial dose: 100 mg extended release orally 3 times a day.
Maintenance dose: 100 mg orally 3 to 4 times a day. If seizure control is established with divided doses of three 100 mg capsules daily, once-a-day dosage with 300 mg of extended release phenytoin sodium may be considered. Alternatively, the dosage may need to be increased up to 200 mg orally 3 times a day, if necessary.
Suspension: Patients who have received no previous treatment may be started on 125 mg (one teaspoonful) of the suspension three times daily, and the dose is then adjusted to suit individual requirements. An increase to five teaspoonfuls daily may be made, if necessary.
IV: Do not exceed the infusion rate of 50 mg/min.
Loading dose: 10 to 15 mg/kg IV slowly.
Maintenance dose: 100 mg IV every 6 to 8 hours.
IM: Avoid the IM route due to erratic absorption.
Usual Adult Dose for Arrhythmias
1.25 mg/kg IV every 5 minutes. May repeat up to a loading dose of 15 mg/kg, or
250 mg orally 4 times a day for 1 day, then 250 mg twice daily for 2 days
300 to 400 mg/day orally in divided doses 1 to 4 times a day
Usual Adult Dose for Status Epilepticus
Loading dose: Manufacturer recommends 10 to 15 mg/kg by slow IV administration (at a rate not exceeding 50 mg/minute). Alternatively, generally accepted guidelines suggest 15 to 20 mg/kg by slow IV administration (at a rate not exceeding 50 mg/minute).
Maintenance rate: 100 mg orally or IV every 6 to 8 hours
Maximum rate: 50 mg/minute
Maintenance dose: IV or Oral: 100 mg every 6 to 8 hours
Usual Adult Dose for Neurosurgery
Neurosurgery (prophylactic): 100 to 200 mg IM at about 4 hour intervals during surgery and the immediate postoperative period. (Note: While the manufacturer recommends IM administration, this route may cause severe local tissue destruction and necrosis. Some clinicians recommend the use of fosphenytoin if IM administration is necessary.) If IM administration is not necessary, accepted protocol has been 100 to 200 mg IV at about 4 hour intervals during surgery and the immediate postoperative period.
What is the dose of phenytoin for a child?
Usual Pediatric Dose for Seizures
Status Epilepticus: Loading Dose:
Infants, Children: 15 to 20 mg/kg IV in a single or divided doses
Anticonvulsant: Loading Dose:
All ages: 15 to 20 mg/kg orally (based on phenytoin serum concentrations and recent dosing history). The oral loading dose should be given in 3 divided doses administered every 2 to 4 hours.
Anticonvulsant: Maintenance Dose:
(IV or oral) (Note: May initially divided daily dose into 3 doses/day, then adjust to suit individual requirements.)
Less than or equal to 4 weeks: Initial: 5 mg/kg/day in 2 divided doses
Usual: 5 to 8 mg/kg/day IV in 2 divided doses (may require dosing every 8 hours).
Greater than or equal to 4 weeks: Initial: 5 mg/kg/day in 2 to 3 divided doses
Usual: (may require up to every 8 hour dosing)
6 months to 3 years: 8 to 10 mg/kg/day
4 to 6 years: 7.5 to 9 mg/kg/day
7 to 9 years: 7 to 8 mg/kg/day
10 to 16 years: 6 to 7 mg/kg/day
Usual Pediatric Dose for Arrhythmias
Greater than 1 year:
Loading Dose: 1.25 mg/kg IV every 5 minutes. May repeat up to a loading dose of 15 mg/kg.
Maintenance Dose: 5 to 10 mg/kg/day orally or IV in 2 to 3 divided doses
How is phenytoin available?
Phenytoin is available in the following dosage forms and strengths:
Capsule. Oral, as sodium: 30 mg; 100 mg; 200 mg; 300 mg;
Solution, Injection, as sodium: 50 mg/mL
Suspension, oral: 125 mg/5 mL (237 mL); 125 mg/5 mL (4 mL, 237 mL)
Tablet Chewable, oral: 50 mg
What should I do in case of an emergency or overdose?
In case of an emergency or an overdose, call your local emergency services or go to your nearest emergency room.
Symptoms of overdose may include:
- uncontrollable eye movements
- loss of coordination
- slow or slurred speech
- uncontrollable shaking of a part of the body
- difficulty understanding reality
- coma (loss of consciousness for a period of time)
What should I do if I miss a dose?
If you miss a dose of phenytoin, take it as soon as possible. However, if it is almost time for your next dose, skip the missed dose and take your regular dose as scheduled. Do not take a double dose.
Hello Health Group does not provide medical advice, diagnosis or treatment.
Review Date: May 30, 2016 | Last Modified: January 4, 2017
Phenytoin. https://www.drugs.com/phenytoin.html. Accessed July 14, 2016.
Phenytoin. http://www.drugbank.ca/drugs/DB00252. Accessed July 14, 2016.
Phenytoin. http://www.webmd.com/drugs/2/drug-4159-1057/phenytoin-oral/phenytoin-suspension---oral/details. Accessed July 14, 2016.