Phenytoin is used to control some types of seizure and other conditions related to the central nervous system. Learn how to use the medication, its common side effects, special precautions to watch out for, and more.

What is this medication for?

Phenytoin is used to control some types of seizure (commonly known as fits) and other conditions related to the central nervous system.

How should I take/use this medication? 

Follow the instructions on your medication label and take the medication as prescribed by your doctor.

You should take this medication with or after food. Take phenytoin two hours apart from a high-protein meal (eg. with milk or meat), otherwise its efficacy may be reduced.

If you are taking the capsules, swallow capsules whole with a glass of water. Do not take capsules that are discoloured.

If you are using oral liquid form/suspension, shake the bottle well before taking. Use a medicine spoon or graduated syringe provided to measure your medication. Do not use household spoons as they may not be accurate.

You may not experience any effect immediately after starting the medication as it may take up to a few weeks before this medication can work fully. You should continue to take your medication regularly as instructed by your doctor even if you feel well. Stopping your medication without informing your healthcare professionals may cause your condition to become worse quickly.

Do not stop taking or adjust the dose of this medication without consulting your healthcare professionals. Do not change brands or formulations of the medication without informing your healthcare professional.

What should I do if I forget to take/use this medication?

If you miss a dose, take the missed dose as soon as you remember. If it is almost time for your next dose, take only the usual dose. Do not double your dose or use extra medication to make up for the missed dose.

What precautions should I take?

Inform your healthcare professionals if you have the following medical conditions before starting on phenytoin:

  • Heart problems or irregular heartbeats
  • Low albumin (a type of protein) in the blood
  • Drug allergies

For female patients of child-bearing age: Discuss with your doctor regarding family planning if you will be starting or currently taking phenytoin. 

Consult your healthcare professional immediately if your seizures get worse or become different after you start on this medication.

What are some common side effects of this medication?

Like all medicines, this medication may cause some possible side effects but not everyone experiences them. Consult your healthcare professional if any of the side effects lasts more than a few days or become severe and bothersome.

The common side effects of phenytoin include:

  • Dizziness
    • Get up slowly from sitting or lying down position
  • Drowsiness
    • Avoid driving, operating machinery or doing strenuous physical activities
    •  Avoid taking over-the-counter medications (such as cold or allergy medications) that could also add on to the drowsiness
  • Headache
    • Take paracetamol, if necessary, to relieve headache
  • Feeling tired, having difficulty in sleeping
  • Nausea or vomiting
    • Take the medicine with or after food to reduce nausea and vomiting
    • Avoid food that is too rich, spicy or has strong smells
    • Take small and frequent meals, rather than 3 large meals
    • Do not take the medication with carbonated drinks
  • Decrease in folic acid level in the body
    • Your doctor may prescribe folic acid supplements for you

If you are taking phenytoin for long term use, some of the following side effects might happen:

  • Growing of hair around the face
  • Thickening of the lips, swollen gums 
    • You can prevent this by maintaining good oral hygiene and using a soft-bristled toothbrush.

What are some rare but serious side effects that I need to seek medical advice immediately?

The symptoms of a drug allergy include one or more of the following:

  • Swollen face / eyes/ lips / tongue
  • Difficulty in breathing
  • Itchy skin rashes over your whole body

If you experience any of these symptoms, you should stop your medication and see your healthcare professional immediately.

If the following serious side effects happen, you should consult your healthcare professionals immediately:

  • Blistering, peeling, red skin rash which can be signs of Stevens-Johnson syndrome (SJS) or toxic epidermal necrolysis (TEN)
    • SJS is a rare and serious skin reaction. Symptoms include face swelling, blisters on skin, skin rashes that spread within hours to days. 
    • TEN is a more severe form of skin reaction. Symptoms include fever and flu-like symptoms, blisters on skin, skin rashes that spread within hours to days. Areas such as the mouth, eyes, genitals are commonly involved. 
    • It usually occurs within the first few months (monitor closely for the first 3 months) of treatment. 
    • As SJS and TEN progress rapidly, please consult healthcare professionals if you show first sign of rash.
  • Fever, chills, sore throat, mouth ulcers or sores
  • Slurred speech, feeling confused
  • Double vision
  • Unstable/ unsteady movements
  • Unexplained bleeding or bruising
  • Sudden decrease in urination
  • Liver problems: Dark-coloured urine or light-coloured stools, yellowing of your eyes or skin, severe loss of appetite, stomach pain, nausea, vomiting that does not go away      

In rare cases, this medication may cause the following changes to a person’s mental condition, especially in the first few weeks of treatment or during dose changes:

  • Worsening agitation, restlessness, violent behaviour, or
  • New or worsening thoughts of harming yourself or ending your life
  • Other changes in mood or behaviour

Please inform your doctor as soon as possible, or for your family or caregiver to inform your doctor if you are unable to seek treatment on your own. 

Do not stop taking this medication on your own without discussing with your doctor.

It is important to note that your doctor has prescribed this medication as he/ she feels you will benefit more from taking this medication over the possible risks that it may cause, which have a low chance of occurring, and most people take this medication without any of such problems.

What food or medication should I avoid when I take/use this medication?

Take antacids 2 hours before or after you take phenytoin as it may reduce absorption of phenytoin. 

If you are on tube feeding, you should take phenytoin 2 hours before or after your feeds.

Phenytoin may decrease the effectiveness of hormonal contraceptives (birth control pills, patches, rings, injections, implants or intrauterine devices) and hepatitis C medications. Discuss with your healthcare professionals regarding other medications while on phenytoin.

Do not take alcohol with this medication as it can make you less alert.

Please consult your healthcare professionals before using any other medications, including over-the-counter medicines, supplements and herbal products as some of these may affect the medication in your body.

How should I store this medication?

Keep the medication in the original container, tightly closed or sealed. Store in a cool and dry place, away from direct sunlight. Keep this medication away from children.

How should I throw this medication safely?

Pack this medication into a black trash bag and seal it tightly before throwing it into the rubbish chute or bin.



Please take note that the above is not a complete list of all possible side effects. If you have any concerns about your medication or if you have other side effects that you think are caused by this medication, please consult your doctor or pharmacist.

If you take more than the recommended dose, please seek medical advice immediately. The information provided on this page does not replace information from your healthcare professional. Please consult your healthcare professional for more information.

This article is jointly developed by members of the National Medication Information workgroup. The workgroup consists of cluster partners (National Healthcare Group, National University Health System and SingHealth), community pharmacies (Guardian, Unity and Watsons) and Pharmaceutical Society of Singapore. The content does not reflect drug availability and supply information in pharmacies and healthcare institutions. You are advised to check with the respective institutions for such information.

Last updated on May 2024 

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