Naomi Carr
Author: Naomi Carr Medical Reviewer: Amy Shelby Last updated:

Phenytoin, also called Dilantin or Phenytek, is an anticonvulsant medication, primarily used in the treatment of epilepsy and various types of seizures. Consult with your doctor before taking any other medications (prescribed or over the counter) while you are on phenytoin, as adverse effects can occur.

Phenytoin brand names

  • Dilantin
  • Phenytek

What is phenytoin prescribed for?

Phenytoin is approved by the US Food and Drug Administration (FDA) for the treatment of epilepsy and various seizures, such as tonic clonic seizures, complex partial seizures, and status epilepticus. [1]

Phenytoin may also be prescribed for off-label uses, which means that it is not FDA approved for these uses but has been deemed necessary for treatment by a medical professional.

Off-label uses for phenytoin include the treatment of various mental health disorders, particularly bipolar disorder. There is very limited research related to the effectiveness and safety of this use of phenytoin, although various small studies have found it to be useful in the treatment of mania and the associated symptoms, such as impulsivity and agitation. [2][3][4][5]

How does phenytoin work?

Phenytoin works by stabilizing certain brain activity, which helps to prevent seizures from occurring, as well as reducing the symptoms of mania, such as impulsivity and agitation.

How is phenytoin usually taken?

Phenytoin is available in extended-release capsules and chewable tablets, and as a liquid. Extended-release medication is slowly released into the body throughout the day, so may require fewer daily doses than immediate-release medication.

Capsules should be swallowed whole, without breaking or crushing.

Chewable tablets can be chewed and swallowed, or swallowed without chewing, if preferred.

Liquid medication should be shaken in the bottle and administered with an appropriate measuring device, such as a syringe. Your doctor will advise how to do this, so be sure to follow their instructions carefully.

Your prescription may depend on your condition, age, weight, and response to the medication.

You may be required to take your medication 1-4 times per day, depending on the form of medication you are prescribed.

Your doctor will likely start your phenytoin prescription on a low dose, slowly increasing it until you reach an effective and therapeutic dose. They will monitor your physical and mental health closely, which may require regular blood tests, particularly at the start of your treatment.

This medication should be taken as prescribed, without missing a dose. If a dose is missed, take the medication as soon as possible, or if it is close to the next dosage time, skip the missed dose. Never take double your prescribed dose of phenytoin in one go, as this can have adverse effects and may increase the risk of side effects.

How long does phenytoin stay in your system?

After you begin taking phenytoin, it may take several weeks to reach a therapeutic level of the medication. You may begin to notice improvements in some of your symptoms within this time. This may depend on the severity of your condition.

After stopping this medication, it may take several weeks for phenytoin to leave your system completely.

Never stop taking phenytoin suddenly or without medical advice, as this can cause withdrawal symptoms. If you doctor advises that it is safe to come off this medication, they will likely reduce your dose slowly to prevent withdrawal symptoms from occurring.

Phenytoin side effects

When you start taking a new medication, you may experience some common side effects. They should reduce within the first week or two, but if they continue, or become problematic, consult with your doctor, as you may need a reduced dose or a change of medication.

Common side effects of phenytoin include:

  • Difficulty falling or staying asleep
  • Abnormal eye or body movements
  • Confusion
  • Slurred speech
  • Headache
  • Enlarged lips and gums
  • Hair growth
  • Constipation
  • Increased blood sugar

Serious side effects of phenytoin are less common but may still occur. [6][7] If you experience any of the following, or any other concerning side effects, seek medical attention immediately:

  • Yellowing of the skin or eyes
  • Allergic reactions, such as rash, blisters, swelling of the mouth or face, tight chest
  • Chest pain or breathing difficulties
  • Unexplained bleeding or bruising
  • Nausea or vomiting
  • Swollen glands
  • Stomach pain
  • Loss of appetite
  • Fever and sweating
  • Irregular heartbeat
  • Changes in mental state, including an increase in depression or anxiety, thoughts of harming yourself or others, aggression, mania, hallucinations, delusions

Phenytoin precautions

Phenytoin can cause suicidal thoughts, particularly at the beginning of your treatment. [6][7] If you or your family notice any concerning changes in your mental state, or you experience any thoughts of harming yourself, contact your doctor or mental health professional immediately.

It is important that your doctor is aware of any past or present mental health conditions you have experienced, to enable safe monitoring of your condition while on this medication, or decide if it is safe for you.

Discuss with your doctor all your past and present physical health conditions, as they may impact your ability to take this medication safely.

If you have liver, heart or kidney problems, diabetes, or any condition resulting in brittle bones, this medication may not be safe for you to take due to the potential side effects. If it is deemed necessary for you to take this medication, your doctor will closely monitor any changes to your physical health to ensure your safety.

Tell your doctor about all medications you are currently taking, or plan to take (including vitamins and dietary supplements), as they may cause adverse reactions.

Inform your doctor if you are pregnant or planning to become pregnant. Phenytoin can cause serious harm to your fetus, so is not considered a safe medication for pregnant women. [1][6] Your doctor can advise you of any safer alternatives. If you are of childbearing age, it may be advised to take contraception while on this medication.

Breastfeeding is considered safe while taking phenytoin, although small amounts of the medication may pass to your baby through breast milk, so caution is advised. You should monitor your baby for any unusual changes in their physical or mental state. [8]

Phenytoin can cause gum and tooth damage, so you may wish to discuss with your doctor the best way to care for your oral health while on this medication.

Phenytoin can make you feel very drowsy, so it is advised not to drive until you know how the medication will affect you and it is safe to do so.

Phenytoin interactions

Some medications may interact with phenytoin, which can cause a decrease in the effectiveness of the medication, or an increased risk of serious side effects.

This includes blood thinners, some antibiotic, antiviral, and antifungal medications, other seizure medications, hormone-based medications (such as hormone replacement therapy and some contraceptives), cholesterol medication, antacids, opioid medications, St. John’s Wort, and some mental health medications, such as antidepressants, antipsychotics, and benzodiazepines.

Always discuss your medications with your doctor prior to commencing a new treatment.

Phenytoin storage

Always keep all medications out of reach of children.

Store phenytoin in its original packaging, in airtight containers, and at room temperature (between 68°F and 77°F).

If you need to dispose of medication that is out of date or no longer needed, contact a medical professional to ensure it is disposed of appropriately. Never flush medications down the toilet or casually deposit them in the garbage bin, as this can create unnecessary risks.

What to do if you overdose on phenytoin

If you overdose on phenytoin, call a medical professional or Poison Control at 1-800-222-1222. In case of an emergency, call 911. A phenytoin overdose can have a serious impact on the central nervous system or heart and can be fatal.

Symptoms of a phenytoin overdose include confusion, vomiting, extreme tiredness, and coma.


Is phenytoin safe to take when pregnant?

Phenytoin is not considered safe to take while pregnant, as it may cause serious harm to your fetus. [1][7] Your doctor can inform you of alternative medications for your condition if you are pregnant or planning to become pregnant.

Are there any alternatives to phenytoin?

There are many alternative medications available to treat mania associated with bipolar disorder, including lithium, other anticonvulsant medications such as carbamazepine and valproate, and antipsychotic medications such as haloperidol, risperidone, and olanzapine, while antidepressant medications can help with depressive episodes. [9][10]

Your doctor can discuss alternative medications with you and advise what may be most suitable. You may also wish to engage in talking therapies, such as cognitive behavioral therapy, counseling, or family therapy, as this can be beneficial to your treatment alongside your medication. [10]

  1. Gupta, M., & Tripp, J. (2022). Phenytoin. In StatPearls [Internet]. Treasure Island, FL: StatPearls Publishing. Retrieved from
  2. Hesselink, J.M.K., & Stahl, S.M. (2018). Phenytoin in bipolar depression: An old chapter, but not yet properly evaluated. Journal of Mood Disorders and Therapy 1(1):24-28. Retrieved from DOI: 10.36959/418/579
  3. Society for Experimental Biology and Medicine. (2010). Antiepileptic drug phenytoin as a mood stabilizer?ScienceDaily. Retrieved from
  4. Mishory, A., Yaroslavsky, Y., Bersudsky, Y., & Belmaker, R.H. (2000). Phenytoin as an antimanic anticonvulsant: A controlled study. The American Journal of Psychiatry157(3), 463–465. Retrieved from
  5. Bersudsky, Y. (2006). Phenytoin: An anti-bipolar anticonvulsant? The International Journal of Neuropsychopharmacology9(4), 479–484. Retrieved from
  6. American Society of Health-System Pharmacists, Inc. (Revised 2019). Phenytoin. MedlinePlus. Retrieved from
  7. Parke-Davis, Pfizer Inc. (Revised 2009). Dilantin (phenytoin sodium) medication guide. Access Data FDA. Retrieved from
  8. Bethesda (MD): National Library of Medicine (US). (2006). Phenytoin. In Drugs and Lactation Database (LactMed) [Internet]. Retrieved from
  9. Geoffroy, P.A., Bellivier, F., & Henry, C. (2014). Traitement du trouble bipolaire en phase maniaque: synthèse critique des recommandations internationales [Treatment of manic phases of bipolar disorder: critical synthesis of international guidelines]. L’Encephale40(4), 330–337. Retrieved from
  10. Geddes, J. R., & Miklowitz, D. J. (2013). Treatment of bipolar disorder. Lancet (London, England)381(9878), 1672–1682. Retrieved from
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Naomi Carr
Author Naomi Carr Writer

Naomi Carr is a writer with a background in English Literature from Oxford Brookes University.

Published: Nov 6th 2022, Last edited: Oct 24th 2023

Amy Shelby
Medical Reviewer Amy Shelby M.S. Counseling Psychology

Amy Shelby is a medical reviewer with a B.A. in Psychology from Northwestern and an M.S. in Psychology from Chatham University.

Content reviewed by a medical professional. Last reviewed: Nov 7th 2022
Medical Reviewer Medical Reviewer:
Amy Shelby
Last reviewed: Nov 7th 2022 Amy Shelby

M.S. Counseling Psychology