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Phenytoin - What is it for

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

Phenytoin - Side Effects, Precautions, and Contraindications

What side effects can Phenytoin cause?

Like all medications, Phenytoin may cause side effects but not everyone experiences them.  

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 medicines (such as cold or allergy medicines) that could also add on to the drowsiness
  • Headache
  • Feeling tired, having difficulty in sleeping
  • 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 face
  • Thickening of the lips, swollen gums
    • You can prevent this by maintaining good oral hygiene and use soft-bristled toothbrush

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

  • Blistering, peeling, red skin rashes which may be signs of Stevens-Johnson syndrome (SJS) or Toxic Epidermal Necrolysis (TEN)
    • SJS and TEN are rare and serious skin reactions. Symptoms include face swelling, blisters on skin, skin rashes that spread within hours to days. 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 your healthcare professional if you show the 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 behavior 

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 such problems.

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.

Before taking Phenytoin , what precautions must I follow?

​Inform your healthcare professional 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 have started this medication.

What food or medicine must I avoid when I take Phenytoin ?

  • ​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). Discuss with your healthcare professional regarding birth control while on Phenytoin.
  • Do not take alcohol with this medication as it can make you less alert.
  • Please consult your healthcare professional before using any other medications, including over-the-counter medications, supplements and herbal products as some of these may affect the medication in your body.

Phenytoin - Dosage and How to Use

How should Phenytoin be used?

  • 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. TakePphenytoin two hours apart from a high-protein meal (e.g. 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 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 works 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 professional may cause your condition to become worse quickly.
  • Do not stop taking or adjust the dose of this medication without consulting your healthcare professional. Do not change the medication that was dispensed to you without informing your healthcare professional.

What should I do if I miss a dose?

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 medicine to make up for the missed dose.

What should I do if I overdose?

Phenytoin - Handling

How should I handle Phenytoin safely?

Phenytoin - Storage

How should I store Phenytoin ?

;#Keep away from children;#Keep in a cool, dry place, away from direct sunlight;#

​Keep the medication in the original container, tightly closed or sealed.

How should I dispose of Phenytoin safely?

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

Phenytoin - Additional Information


  • Updated on 3/31/2022 12:00:00 AM
  • Article contributed by PSS National Medication Information Workgroup PSS National Medication Information Workgroup
The information provided is not intended as medical advice. Terms of use. Information provided by SingHealth

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