Prescription vs OTC: Prescription by Doctor required
Last Updated: March 22, 2022

Phenytoin is prescribed for the management of generalized tonic-clonic (grand mal), complex partial seizures. It is also used for the prevention of seizures following head trauma/neurosurgery.

Phenytoin is an anti-seizure medication. It is used to prevent partial seizures, tonic-clonic seizures, but not absence seizures. It may also be used to treat certain heart arrhythmias or neuropathic pain. It can be taken intravenously or by mouth.

Common side effects include nausea, increased hair growth, stomach pain, poor coordination, loss of appetite, and enlargement of the gums. Potentially serious side effects include self harm, sleepiness, liver problems, low blood pressure, bone marrow suppression and toxic epidermal necrolysis.

Phenytoin should be used with caution in patients having low blood pressure, heart failure, liver disease, suicidal tendencies, kidney disorders, vitamin D deficiency, diabetes and porphyria. It should not be used in people having allergy towards Phenytoin or other ingredients in the medicine or people suffering from certain heart conditions. There is evidence that use during pregnancy results in abnormalities in the baby. It appears to be safe to use during breastfeeding.

Take this medication orally as per your doctor’s prescription, with plenty of water. Avoid consumption of alcohol during the treatment. If for some reason you cannot take the medicine orally, it may be administered by a healthcare professional as an injection into a muscle or a vein, or by diluting it and giving it as a drip into a vein.

The intravenous form generally begins working within 30 minutes and is effective for 24 hours. Blood levels of the patient can be measured to determine the suitable dose of the medicine.

Use Phenytoin cautiously in malnutrition (more free drug circulating), significant hepatic disease (adjust dose), or with other highly protein-bound drugs. Phenytoin is used cautiously in hypotension (I.V. form contains propylene glycol). I.V. form may cause hypotension.

Information given here is based on the salt content of the medicine. Uses and effects of the medicine may vary from person to person. It is advisable to consult a Neurologist before using this medicine.

This medicine is primarily used to treat the following conditions:

  • Generalized Tonic-Clonic Seizure

  • Seizures

This medicine can cause side effects that includes:

  • Is It safe with alcohol?

    Phenytoin may cause excessive drowsiness and calmness with alcohol.
  • Are there any pregnancy warnings?

    Phenytoin is unsafe to use during pregnancy.
    There is positive evidence of human fetal risk, but the benefits from use in pregnant women may be acceptable despite the risk, for example in life-threatening situations. Please consult your doctor.
  • Are there any breast-feeding warnings?

    Phenytoin is safe to use during lactation. \nHuman studies have shown that either the drug does not pass into the breastmilk in significant amount or is not expected to cause toxicity to the baby.
  • Is it safe to drive while on this medicine?

    Phenytoin may make you feel dizzy, sleepy, tired, or decrease alertness. If this happens, do not drive.
  • Does this affect kidney function?

    Phenytoin is probably safe to use in patients with kidney disease. Limited data available suggests that dose adjustment of Phenytoin may not be needed in these patients. Please consult your doctor.
  • Does this affect liver function?

    Phenytoin should be used with caution in patients with liver disease. Dose adjustment of Phenytoin may be needed. Please consult your doctor.
  • What is the onset of action?

    The effect of this medicine will be initiated within 30 minutes to 1 hour after its administration.
  • How long is the duration of effect?

    The effect of this medicine will last for about 8 - 10 hours.
  • Missed Dose instructions

    Take the missed dose as soon as you remember. If it’s almost time for your next dose, skip the missed dose. Do not double your dose to make up for the missed dose.
  • Overdose instructions

    Seek emergency medical treatment or contact the doctor if any symptoms of the overdose like uncontrolled body movements, slurred speech appear.
  • India

  • United States

  • Japan

Below is the list of medicines, which contains Phenytoin as ingredient

Phenytoin is an anticonvulsant which is known to prevent seizures by causing a voltage-dependent block of voltage gated sodium channels. It acts by increasing sodium efflux from the neurons of the motor cortex which reduces the post-tetanic potentiation at synapses. This inhibits the cortical seizure foci from spreading to adjacent areas, thus stabilizing the threshold against hyperexcitability.

Whenever you take more than one medicine, or mix it with certain foods or beverages, you"re at risk of a drug interaction.

  • Interaction with Alcohol

    • Alcohol: Additive CNS depression has been reported with hydantoins.
    • Alcohol (acute use): Inhibits metabolism of phenytoin; avoid or limit use; watch for sedation.
    • Alcohol (chronic use): Stimulates metabolism of phenytoin; avoid or limit use.
  • Interaction with Lab Test

    This medicine may interact and can cause an increase in glucose, alkaline phosphatase (S) and decrease in thyroxine (S), calcium (S) levels.
  • Interaction with Medicine

    • Phenytoin may decrease the effect of oral contraceptives, itraconazole, mebendazole, methadone, oral midazolam, valproic acid, cyclosporine, theophylline, doxycycline, quinidine, mexiletine, disopyramide.
    • Amiodarone or disulfiram decreases metabolism of Phenytoin.
    • Isoniazid, chloramphenicol, or fluconazole may increase Phenytoin serum concentrations.
    • Valproic acid may increase, decrease, or have no effect on Phenytoin serum concentrations.
    • Dopamine: Phenytoin may increase the effect of dopamine (enhanced hypotension).
    • Primidone: Increase the rate of conversion of to phenobarbital resulting in increased phenobarbital serum concentrations.
    • Ticlopidine increases serum phenytoin concentrations to increase toxicity of Phenytoin.
    • Acetaminophen: Phenytoinmay enhance the hepatotoxic potential.
    • Acetazolamide: Concurrent use with Phenytoin may result in an increased risk of osteomalacia.
    • Phenytoin may increase the metabolism of alprazolam, amiodarone, bromfenac, carbamazepine, clozapine, cyclosporine, diazepam, disopyramide, doxycycline, felbamate, furosemide, itraconazole, lamotrigine, mebendazole, meperidine, methadone, metyrapone, mexiletine, midazolam, oral contraceptives, quetiapine, quinidine, tacrolimus, teniposide, theophylline, thyroid hormones, triazolam, and valproic acid resulting in decreased levels/effect.
    • Trimethoprim, sulfamethoxazole, valproic acid, sulfamethizole, sulfaphenazole, trimethoprim, nifedipine, omeprazole, phenylbutazone, phenobarbital, amiodarone, chloramphenicol, cimetidine, ciprofloxacin, disulfiram, enoxacin, norfloxacin, felbamate, fluconazole, fluoxetine, influenza vaccine, isoniazid, and metronidazole inhibit the metabolism of Phenytoin resulting in increased serum phenytoin concentrations/effects; monitor.
    • Carbamazepine, cisplatin, diazoxide, ethanol (chronic), folic acid, phenobarbital, pyridoxine, and rifampin may enhance the metabolism of Phenytoin resulting in decreased serum concentrations.
    • Concurrent use of I.V. Phenytoin with dopamine may result in an increased risk of hypotension.
    • Phenytoin may inhibit the anti-Parkinson effect of levodopa.
    • Concurrent use of Phenytoin and lithium has resulted in lithium intoxication.
    • Phenytoin enhances the conversion of primidone to phenobarbital resulting in elevated phenobarbital serum concentrations.
    • Sucralfate may reduce the GI absorption of Phenytoin.
    • Valproic acid and sulfisoxazole may displace Phenytoin from binding sites.
    • Vigabatrin and theophylline may reduce Phenytoin serum concentrations.
    • Warfarin: Phenytoin transiently increased the hypothrombinemia response to warfarin initially; this is followed by an inhibition of the hypoprothombinemic response.
  • Interaction with Food

    • Folic acid: Low erythrocyte and CSF folate concentrations. Phenytoin may decrease mucosal uptake of folic acid; to avoid folic acid deficiency and megaloblastic anemia, some clinicians recommend giving patients on anticonvulsants prophylactic doses of folic acid and cyanocobalamin.
    • Calcium: Hypocalcemia has been reported in patients taking prolonged high-dose therapy with an anticonvulsant. Phenytoin may decrease calcium absorption. Monitor calcium serum concentration and for bone disorders (eg, rickets, osteomalacia). Some clinicians have given an additional 4,000 Units/week of vitamin D (especially in those receiving poor nutrition and getting no sun exposure) to prevent hypocalcemia.
    • Vitamin D: Phenytoin interferes with vitamin D metabolism and osteomalacia may result; may need to supplement with vitamin D.
    • Glucose: Hyperglycemia and glycosuria may occur in patients receiving high-dose therapy. Monitor blood glucose concentration, especially in patients with impaired renal function.
    • Tube feedings: Tube feedings decrease phenytoin bioavailability; to avoid decreased serum levels with continuous NG feeds, hold feedings for 2 hours prior to and 2 hours after phenytoin administration, if possible. There is a variety of opinions on how to administer phenytoin with enteral feedings. BE CONSISTENT throughout therapy.

Ques: What is Phenytoin?

Ans: Phenytoin is a salt which performs its action by reducing the abnormal and excessive activity of the nerve cells in the brain. Phenytoin is used to treat conditions such as Seizures and Status Epilepticus.

Ques: What are the uses of Phenytoin?

Ans: Phenytoin is a medication, which is used for the treatment and prevention of conditions such as Migraine and Cardiac Arrhythmias. Apart from these, it can also be used to treat conditions like Seizures and Status Epilepticus. The patient should inform the doctor about any ongoing medications and treatment before using Phenytoin to avoid undesirable effects.

Ques: What are the Side Effects of Phenytoin?

Ans: Phenytoin is a salt that has some commonly reported side effects. These side effects may or may not occur always and some of them are rare but severe. This is not a complete list and if you experience any of the below-mentioned side effects, contact your doctor immediately. Here are some side effects of Phenytoin which are as follows: Decreased coordination, Nervousness, Unsteadiness, Shaking of hands or feet, Unusual facial expressions, Trouble sleeping, Uncontrolled eye movements, Yellow-colored eyes or skin, Skin rash, and Headache. It is a list of possible side-effects that may occur due to the constituting ingredients of Phenytoin.

Ques: What are the instructions for storage and disposal Phenytoin?

Ans: Phenytoin should be kept in a cool dry place and in its original packaging. Make sure this medication remains unreachable to children and pets. The patient should consult a doctor for its further uses and side effects and should inform the doctor about any ongoing medications and treatment before using to avoid undesirable effects. It is a prescribed medication.

Ques: Should I use Phenytoin empty stomach, before food or after food?

Ans: This medication is to be consumed orally. The salts involved in this medication react properly if it is taken with the food. If you take it on an empty stomach, it might upset your stomach. Please consult the doctor before using Phenytoin.

Ques: How long do I need to use Phenytoin before I see improvement of my conditions?

Ans: The time taken by this medication to improve your health is unknown. Thus it is advised to use this medication for the time period it is prescribed by your doctor. Using this medication longer than its prescribed time may cause some side effects to the patient. Please consult your doctor before using Phenytoin.

Ques: Is there any food or drink I need to avoid?

Ans: You can follow your normal diet under the usage of this medication. Avoidance of any specific food product is not required. Though staying away from practices like drinking and smoking can improve your health.

Ques: Will Phenytoin be more effective if taken in more than the recommended dose?

Ans: Excess of usage of this medication can trigger side effects. The salts involved may cause inadequate effects to your health and can damage your liver if Phenytoin is taken more than its prescribed dosage or taken more times than its prescribed frequency.

Ques: Can I take Phenytoin with ibuprofen?

Ans: This drug can be taken by ibuprofen. It is also advised to consult your doctor before consuming it.

Ques: Does Phenytoin cause weight gain?

Ans: This drug has not been reported to cause weight gain. However, weight loss can occur with long term use of a higher dose of Phenytoin. Please consult your doctor if you experience weight gain after taking Phenytoin.

Ques: Does Phenytoin affect birth control?

Ans: Yes, it does affect birth control. For better understanding, please consult your doctor.
Disclaimer: The information produced here is best of our knowledge and experience and we have tried our best to make it as accurate and up-to-date as possible, but we would like to request that it should not be treated as a substitute for professional advice, diagnosis or treatment.

Lybrate is a medium to provide our audience with the common information on medicines and does not guarantee its accuracy or exhaustiveness. Even if there is no mention of a warning for any drug or combination, it never means that we are claiming that the drug or combination is safe for consumption without any proper consultation with an expert.

Lybrate does not take responsibility for any aspect of medicines or treatments. If you have any doubts about your medication, we strongly recommend you to see a doctor immediately.

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Content Details
Written By
PhD (Pharmacology) Pursuing, M.Pharma (Pharmacology), B.Pharma - Certificate in Nutrition and Child Care
Reviewed By
MBBS, Master in Healthcare Administration, Diploma in Occupational Health
General Physician
Table of Content
About Phenytoin
What are the uses of Phenytoin
What are the contraindications of Phenytoin ?
What are the side effects of Phenytoin ?
Key highlights of Phenytoin
What are the dosage instructions?
Where is the Phenytoin approved?
Medicines containing Phenytoin
How does this medication work?
What are the interactions of Phenytoin ?
FAQs about Phenytoin