Phenytoin (Oral)

Phenytoin is an anticonvulsant that is used to prevent seizures from occurring and is employed both during brain surgery and for the treatment of epilepsy.


Phenytoin is an anticonvulsant medication that is used to prevent seizures. It acts upon the brain tissue to stop the occurrence of seizures.

Typically Phenytoin is used to prevent seizures from occurring in patients with epilepsy, but it is also used during brain surgery to prevent seizures from occurring.

The medication is available only with a prescription and comes in a range of forms including: capsules, extended release capsules, chewable tablets and suspension.

Condition treated

  • Epilepsy, seizure
  • Seizure during brain surgery

Type Of Medicine

  • Anticonvulsant

Side Effects

In addition to the desired anticonvulsive effects from the use of Phenytoin, other side effects that are unwanted can also occur. Patients should familiarize themselves with these potential side effects before beginning use of this treatment. While not all side effects may occur, in some cases they may require that the patient seeks medical attention.

Patients who experience the following side effects should seek immediate medical attention:

Occurring commonly:

  • Decrease in coordination
  • Decreased muscle control or coordination
  • Mental confusion or disorientation
  • Nervousness
  • Slurring of speech
  • Trembling
  • Trouble with breathing, speaking or swallowing
  • Unsteadiness

Occurring only rarely:

  • Inability to move the eyes
  • Increased blinking or spasming of the eyelid
  • Muscle twitching or twisting
  • Protrusion of the tongue
  • Shakiness and unsteadiness when walking
  • Shaking of the legs, arms, hands or feet
  • Trembling of the hands or feet
  • Uncontrolled movements of the neck, trunk, arms or legs
  • Uncontrolled movements of the tongue, lips, face, arms or legs
  • Unusual facial expressions

Frequency of occurrence not known:

  • Abdominal pains
  • Black stools with a tar-like appearance
  • Bleeding of the gums
  • Blistering or peeling of the skin
  • Blisters
  • Bloating of the stomach area
  • Blood appearing in the urine or stools
  • Bloody stools
  • Chest pains
  • Chills
  • Coughing
  • Darkened urine
  • Diarrhea
  • Difficulty experienced when moving
  • Fainting spells
  • Feverishness with or without chills
  • General feeling of discomfort
  • General feeling of illness
  • General feeling of tiredness
  • General feeling of weakness
  • Hair loss
  • Headache
  • High fever
  • Hives
  • Hoarseness
  • Irregularity of heartbeat
  • Irritated eyes
  • Irritation in the mouth
  • Itching
  • Joint pains
  • Lightened coloration of stools
  • Loosening of the skin
  • Lower back pain
  • Muscle pain
  • Muscle stiffness
  • Nausea
  • Numbness, tingling or pain experienced in the hands or feet
  • Pain or difficulty experienced during urination
  • Paleness of the skin
  • Red and swollen gums
  • Red spots appearing on the skin
  • Side pain
  • Skin blisters
  • Skin lesions appearing that often have a purple center
  • Skin rash
  • Soreness of the throat
  • Sores and ulcers appearing around the mouth
  • Stomach pain
  • Swelling or pain in the lymph glands of the neck, armpit or groin
  • Unusual bleeding
  • Unusual bruising
  • Unusual tiredness
  • Upper right abdominal or stomach pains
  • Vomiting
  • Weakness
  • Weight loss
  • White spots appearing on the lips or in the mouth
  • Yellowing of the eyes or skin

Other side effects from use can occur that would not normally require that you seek medical attention. If the following symptoms do, however, become bothersome or are ongoing then you can consult your doctor for advice on how best to alleviate them:

Occurring commonly:

  • Troubled sleep
  • Uncontrolled movements of the eye

Frequency of incidence not known:

  • Difficulty passing a bowel movement
  • Enlargement of the lips
  • Feeling that self or surroundings are constantly moving
  • Increased hair growth on the forehead, back, arms or legs
  • Pain experienced during erection of the penis
  • Sensation of spinning of self or surroundings

Other side effects can also occur that have not been listed herein. If you notice any other side effects then you should contact your healthcare professional for advice on what to do. If you believe that your immediate health is at risk, however, then you should always contact the emergency services.


This medication should be taken only as you have been directed to by your doctor. You should not take it more frequently nor take it for longer than you have been ordered to by your doctor. You should never take an increased dose of this medication.

Phenytoin should always come with a medication guide that you should read and ensure that you understand. If you do not understand any of it then you should check with your doctor before use.

Phenytoin can be used in conjunction with other medications for the prevention of seizures. If you are taking other treatments then you must inform your doctor and should continue to do so unless otherwise ordered.

You should not change brand of Phenytoin without talking to your doctor as the different brands work differently and will have unusual effects on your body. If your medication looks to be different when you go for a refill then you should consult your pharmacist.

Each tablet should be taken with a glass of water. When using the suspension medication you should use the appropriately marked spoon, syringe or cup to ensure that you are getting the correct dose of the treatment.

The dose of this medication will differ for each patient and you should only take it as directed by your doctor. The following information is a guideline only and should not be used to change your dose. Only change your dose when directed to by your doctor:

When treating seizures with extended release tablets:

  • Adults should begin with a dose of 100mg taken thrice daily or 300mg taken once daily. The dose may be increased as required.
  • Adults in a hospital environment may be given a dose of 1000mg split across 3 doses given every 2 hours. After 24 hours from this loading dose the patient can be placed onto a normal maintenance dose.
  • Teenagers and children aged 6+ may be given a dose of 300mg per day. The dose may be adjusted as required.
  • Below the age of 6 the dose will be determined by the doctor based on the body weight of the child. The usual dose is 5mg/kg divided into 2-3 doses per day. This dose may be adjusted as required.

When treating seizures with suspension medication:

  • Adults should begin by taking 5ml 1-3 times daily. The doctor may adjust this dose as required.
  • Teenagers and children aged 6+ should be given 300mg per day initially. This dose may be adjusted as required.
  • Children aged below 6 years will be given a dose based on body weight that is normally 5mg/kg split into 2-3 doses. The doctor may adjust this dose as is required.

When treating seizures with tablets:

  • Adults should begin with 100mg taken thrice daily. The doctor may make changes to this dose as required.
  • Teenagers and children over the age of 6 may be given 100mg 2-13 times daily. The doctor may make adjustments to this dosing as required.
  • Children aged below 6 will be given a dose of 5mg/kg of body weight as determined by the doctor. This dose is divided into 2-3 doses and may be adjusted by the doctor as required.

If a dose of Phenytoin is missed then you should take it as soon as possible. If you are close to your next dose being due, however, then you should skip the missed dose completely. You should not take double doses or increase frequency to make up for missing a dose.


Certain medications should never be used in conjunction with one another. Doing so can expose the patient to significant risk because drugs can interact with one another in the body. While Phenytoin can be used with a range of anticonvulsants there are also a large number of medications that it cannot be used in conjunction with. Before beginning use you must inform your doctor of all medications that you are taking so that they may ensure no interactions will occur. This includes all prescribed treatments, over the counter medications, herbal remedies and any dietary supplements you may be taking too. This is especially important in the following cases.

Phenytoin is not recommended for use in conjunction with the following treatments. Your doctor may decide against treatment with this medication:

  • Telaprevir
  • Ritonavir
  • Rilpivirine
  • Ranolazine
  • Praziquantel
  • Piperaquine
  • Paritaprevir
  • Ombitasvir
  • Maraviroc
  • Lurasidone
  • Isavuconazonium
  • Grazoprevir
  • Elvitegravir
  • Elbasvir
  • Delavirdine
  • Delamanid
  • Dasabuvir
  • Darunavir
  • Daclatasvir
  • Cobicistat
  • Boceprevir
  • Atazanavir
  • Artemether
  • Amifampridine

The use of Phenytoin is not normally recommended in patients who are also taking the following medications. In some cases, however, both treatments may still be the best treatment available for you. Your doctor may prescribe both but may make changes to the dose or frequency of use of one or more of the treatments.

  • Vortioxetine
  • Voriconazole
  • Vorapaxar
  • Vinflunine
  • Vincristine Sulfate Liposome
  • Vincristine
  • Vilazodone
  • Venetoclax
  • Vemurafenib
  • Velpatasvir
  • Vandetanib
  • Ulipristal
  • Trabectedin
  • Tolvaptan
  • Tofacitinib
  • Ticagrelor
  • Thiotepa
  • Theophylline
  • Tenofovir Alafenamide
  • Temsirolimus
  • Tasimelteon
  • Tamoxifen
  • Tacrolimus
  • Sunitinib
  • St John's Wort
  • Sorafenib
  • Sonidegib
  • Sofosbuvir
  • Simeprevir
  • Siltuximab
  • Sertraline
  • Secukinumab
  • Saquinavir
  • Romidepsin
  • Rolapitant
  • Roflumilast
  • Rocuronium
  • Rivaroxaban
  • Rifampin
  • Reserpine
  • Regorafenib
  • Quetiapine
  • Posaconazole
  • Ponatinib
  • Pixantrone
  • Perampanel
  • Pazopanib
  • Panobinostat
  • Palbociclib
  • Oxycodone
  • Osimertinib
  • Orlistat
  • Olaparib
  • Nintedanib
  • Nimodipine
  • Nilotinib
  • Nifedipine
  • Netupitant
  • Naloxegol
  • Mifepristone
  • Miconazole
  • Methotrexate
  • Manidipine
  • Magnesium Trisilicate
  • Magnesium Oxide
  • Magnesium Hydroxide
  • Magnesium Carbonate
  • Magaldrate
  • Macitentan
  • Lopinavir
  • Linagliptin
  • Lidocaine
  • Ledipasvir
  • Lapatinib
  • Ketoconazole
  • Ixazomib
  • Ixabepilone
  • Ivacaftor
  • Ivabradine
  • Itraconazole
  • Irinotecan Liposome
  • Irinotecan
  • Infliximab
  • Imatinib
  • Ifosfamide
  • Idelalisib
  • Ibrutinib
  • Hydrocodone
  • Halothane
  • Golimumab
  • Gefitinib
  • Fosaprepitant
  • Fluvastatin
  • Fentanyl
  • Ezogabine
  • Exemestane
  • Everolimus
  • Etravirine
  • Ethosuximide
  • Eslicarbazepine Acetate
  • Erlotinib
  • Enzalutamide
  • Eliglustat
  • Dronedarone
  • Doxorubicin Hydrochloride Liposome
  • Doxorubicin
  • Dolutegravir
  • Dihydroxyaluminum Sodium Carbonate
  • Dihydroxyaluminum Aminoacetate
  • Digoxin
  • Diazoxide
  • Diazepam
  • Deferasirox
  • Dasatinib
  • Dabrafenib
  • Dabigatran Etexilate
  • Cyclophosphamide
  • Crizotinib
  • Cobimetinib
  • Clozapine
  • Clarithromycin
  • Ceritinib
  • Cariprazine
  • Carbamazepine
  • Calcium Carbonate
  • Calcifediol
  • Cabozantinib
  • Bupropion
  • Brexpiprazole
  • Bosutinib
  • Bortezomib
  • Blinatumomab
  • Bedaquiline
  • Beclamide
  • Axitinib
  • Aripiprazole
  • Aprepitant
  • Apremilast
  • Apixaban
  • Apazone
  • Aluminum Phosphate
  • Aluminum Hydroxide
  • Aluminum Carbonate, Basic
  • Afatinib
  • Abiraterone

The use of Phenytoin with the following treatments will increase your risk of suffering from certain side effects. Patients may be prescribed both treatments but the doctor may adjust the dose or frequency of use of one or the other.

  • Viloxazine
  • Vigabatrin
  • Verapamil
  • Vecuronium
  • Valproic Acid
  • Valacyclovir
  • Tubocurarine
  • Trimethoprim
  • Triamcinolone
  • Trazodone
  • Topiramate
  • Tolbutamide
  • Tizanidine
  • Tirilazad
  • Ticrynafen
  • Ticlopidine
  • Tiagabine
  • Tenidap
  • Telithromycin
  • Tegafur
  • Sulthiame
  • Sulfaphenazole
  • Sulfamethoxazole
  • Sulfamethizole
  • Sirolimus
  • Simvastatin
  • Shankhapulshpi
  • Sabeluzole
  • Rufinamide
  • Risperidone
  • Rifapentine
  • Remacemide
  • Quinine
  • Quinidine
  • Progabide
  • Prednisone
  • Prednisolone
  • Piperine
  • Phenprocoumon
  • Paroxetine
  • Pancuronium
  • Paclitaxel
  • Oxcarbazepine
  • Ospemifene
  • Norgestrel
  • Norgestimate
  • Norethindrone
  • Norelgestromin
  • Nisoldipine
  • Nilutamide
  • Nelfinavir
  • Nafimidone
  • Midazolam
  • Methsuximide
  • Methoxsalen
  • Mestranol
  • Meperidine
  • Medroxyprogesterone Acetate
  • Levothyroxine
  • Levonorgestrel
  • Levomethadyl
  • Levodopa
  • Isoniazid
  • Imipramine
  • Ginkgo
  • Fosamprenavir
  • Folic Acid
  • Fluvoxamine
  • Fluoxetine
  • Fluorouracil
  • Fludrocortisone
  • Fluconazole
  • Felbamate
  • Etonogestrel
  • Ethynodiol Diacetate
  • Ethinyl Estradiol
  • Estradiol Valerate
  • Estradiol Cypionate
  • Drospirenone
  • Doxifluridine
  • Doxepin
  • Disulfiram
  • Disopyramide
  • Diltiazem
  • Digitoxin
  • Dienogest
  • Dicumarol
  • Dexamethasone
  • Desogestrel
  • Cyclosporine
  • Cortisone
  • Colesevelam
  • Clopidogrel
  • Clofazimine
  • Clobazam
  • Cisplatin
  • Ciprofloxacin
  • Cimetidine
  • Chloramphenicol
  • Caspofungin
  • Carboplatin
  • Capecitabine
  • Busulfan
  • Bleomycin
  • Bexarotene
  • Betamethasone
  • Atorvastatin
  • Amprenavir
  • Amitriptyline
  • Amiodarone
  • Acyclovir
  • Acetazolamide
  • Acetaminophen

Phenytoin can also react with other treatments that have not been listed herein. This list has been compiled due to the severity of interactions but you should ensure that your doctor is aware of all treatments you are taking before beginning use.

Foods and drinks can also interact with medications that you use and the consumption of everyday foodstuffs can impact on certain medications. In the cases of Phenytoin, the following should be avoided:

  • Enteral nutrition
  • Ethanol (alcohol)

If you have any concerns about this then you should discuss this with your doctor.

Certain medical problems can also impact the use of certain medications. This can be because the condition will be worsened by treatment or because the condition will affect the delivery of the treatment. You must inform your doctor of all medical conditions from which you suffer before beginning use of this treatment. This is especially important if you suffer from any of the following:

Phenytoin may make these conditions worse:

The effects of Phenytoin may be intensified due to slower removal from the body in patients with the following conditions:

Other medical conditions can also impact the use of Phenytoin. Ensure your doctor is aware of your full medical history.


Before beginning use of Phenytoin it is important that you understand the risks of using this medication and have weighed them against the benefits of doing so. Before use you should ensure that you have read and understood the medication guide. You should then decide with your doctor whether this is the best treatment for you having taken all of the following factors into consideration:

Allergies It is vital that you inform your doctor if you have ever reacted badly to any medications in the past. Allergies to medications or to any other substances could prohibit use of this treatment.

Pediatric The are no known limitations to the use of this treatment in children.

Geriatric There is no known limitation to the use of this treatment in elderly patients. Elderly patients will, however, be more likely to suffer from age-related conditions that may limit the use of this treatment. These include liver, heart and kidney problems that may require changes to dosing to be made.

Pregnancy You must inform your doctor before use if you are pregnant. There are significant risks to the use of this treatment when pregnant. If you may get pregnant or are planning to get pregnant during use then you must discuss this with your doctor. Use of effective birth control should be practiced to prevent pregnancy during use. You should inform your doctor immediately if you believe that you are pregnant during use.

Breastfeeding There are no adequate studies that determine the risk to an infant through breastfeeding while the mother is taking Phenytoin. You should discuss the risk with your doctor and weigh it against the benefits of breastfeeding.

When using this medication the doctor will need to see the patient regularly to check their progress and to look for any occurrence of unwanted side effects. This may require that they take blood tests.

You should not use this treatment in conjunction with delavirdine under any circumstances. Inform your doctor if you use this treatment.

It is possible that problems of the lymph nodes may occur during treatment with Phenytoin. You must inform your doctor straight away if any of the following symptoms of this occur:

  • Swelling of pain in the lymph glands of:
    • Armpit
    • Groin
    • Neck

You should not cease use of this treatment abruptly. If you wish to stop use then you must inform your doctor and you must reduce the dose gradually. Stopping the medication suddenly can cause seizures and can put the patient at significant risk. Your doctor will develop a plan to gradually reduce treatment if you are going to stop use.

Allergic reactions may occur during use and should be reported immediately. Contact your doctor if any of the following develop:

  • Skin rash
  • Hives
  • Swelling

A life-threatening condition called drug reaction with eosinophilia systemic symptoms (DRESS) can occur during the use of this treatment. The following symptoms must be reported to your doctor immediately if it occurs:

  • Feverishness
  • Rash
  • Swelling
  • Pain
  • Tenderness of the lymph glands
  • Unusual bruising
  • Unusual bleeding
  • Yellowing of the eyes
  • Yellowing of the skin

Liver damage can occur from the use of Phenytoin. You must inform your doctor straight away if you experience any of the following symptoms of liver damage:

  • Stomach pains
  • Pale stools
  • Darkened urine
  • Lost appetite
  • Nausea
  • Vomiting
  • Yellowing of the skin
  • Yellowing of the eyes

Phenytoin may lower the number of white blood cells in the patient's blood and this can increase the risk of infection. White blood cells would normally fight infection in the body but may not be able to with a lowered count. There may also be a reduction in platelets in the blood which increases the risk of bleeding. Platelets normally help the blood to clot if there is a bleed. To limit the risk of infection and/or bleeding patients should follow the following advice during use of Phenytoin:

  • Avoid people with infections wherever possible. It is best to avoid busy public places and enclosed spaces where people may be spreading infection. Avoid public transport where you can.
  • Check with your doctor if you show any signs of infection including a cough or cold.
  • Inform your doctor straight away if you notice any unusual bruising or bleeding or any changes to stools or urine.
  • Seek oral advice from your doctor. Brushes, floss and picks may cause bleeding so alternative means of maintaining oral hygiene may need to be found.
  • Only touch your eyes, nose or ears if you have washed your hands and face thoroughly immediately before use.
  • Take extra care not to cut yourself when trimming nails, shaving or cutting vegetables. You should be extremely cautious with sharp objects.
  • Avoid any situations where you are likely to get injured. This includes contact sports and situations in which you are likely to slip or fall.

Patients may experience a decrease in bone density during treatment and this may put the patient at risk of experiencing certain problems such as weak bones or osteoporosis. If you have any questions then you should discuss this with your doctor.

Phenytoin can cause agitation, irritability or abnormality of behavior in some patients. In some it may also cause an increase in occurrence or suicidal thoughts. If you notice any of these then it is very important that you inform your doctor, particularly if you have a tendency to become depressed.

Problems with the gums may occur during use. Ensuring that you maintain a high standard of oral hygiene will help you to prevent this from occurring. Your mouth should be kept as clean as possible and you should visit a dentist for cleaning every 6 months. If you have questions about how best to clean your mouth and gums you should consult your dental practitioner.

You must inform any medical professional that you visit that you are taking this medication before undergoing any tests or treatment. Phenytoin may change the results of some tests and may lead to a misdiagnosis. Use may also prohibit certain procedures and may prohibit the use of certain medications for treatment.

Phenytoin can cause the patient to become drowsy, dizzy, inattentive or experience a lack of coordination. Patients should not drive or operate machinery in this state because it will put them at significant risk of causing a dangerous accident. Remember that it is your responsibility to ensure that you are safe to drive or operate machinery at all times before doing so. Do not put others at risk through your use of this treatment.

Patients taking Phenytoin should not drink alcohol. If this concerns you then you should discuss this with your doctor before beginning treatment.

You should not take any other medications with this treatment unless they have been discussed with your doctor. Phenytoin can be taken with other anticonvulsants and you should not stop use unless directed to, but you must ensure you have discussed this use with your doctor before continuing.


You should store your medication at room temperature. Keep Phenytoin away from direct sunlight, moisture and heat.

All medications should be kept out of the reach of children at all times.

You should not keep outdated medication for longer than required and you should ensure that all unwanted, outdated and unneeded medication is disposed of safely as directed by your healthcare professional.


Phenytoin is an anticonvulsant medication that is used in the treatment of seizures. It is predominantly used in patients who suffer seizures because of epilepsy. Phenytoin is also used to prevent the occurrence of seizures in patients undergoing brain surgery. Phenytoin acts on the brain tissue to prevent the occurrence of seizures.

Phenytoin is prescribed for daily use in 1-3 doses to be taken daily. Patients should follow the exact directions for use given by their doctor. Only regular use will ensure efficient and safe treatment of the condition and cessation of use or missed doses will expose the patient to a significant risk of seizures occurring.

A wide range of side effects can occur from the use of Phenytoin that can require medical attention in some cases. Some side effects may also cause life-threatening conditions. The patient should ensure that they understand these risks before use and must consider the benefits of use against the potential risks occurring from use.

This medication is issued with a medication guide. Patients should read and understand this guide before beginning use and should ask their doctor if there is anything that they do not understand.

Patients using Phenytoin should avoid the consumption of alcohol at all times. You should discuss this with your doctor if this is a problem for you.