Phenytoin information sheet

• Phenytoin is best taken at approximately the same time of day, preferably with food or following meals

• Chewable tablets may be swallowed whole, chewed or crushed before swallowing

• Suspension needs to be shaken well before use. Measure dose accurately with syringe or measuring spoon. 

If this drug is given through a feeding tube:

• Stop tube feeding 2 hours before giving this drug if possible

• Delay feed for 2 hours after giving the drug

• Please discuss this with your doctor if continuous feed is necessary

There are many drugs including other anti-epileptics that interact with phenytoin and can alter its levels in the blood. 

Phenytoin can also change the level of other medications in the blood so always tell your doctor that your child is taking phenytoin. This is also important if you are giving your child herbal or complementary medicines. The doctor can check to make sure they are compatible. 

Monitoring and medication levels 

Levels of phenytoin can be unpredictable and need to be monitored in the blood, especially when the medication is started and with any changes in dose. Phenytoin levels are usually taken before the medication is due (trough level), and as advised by your doctor. 

• If your child vomits within 30 minutes after having their dose, then repeat the dose.
• If it is longer than 30 minutes since having the dose you do not need to repeat the dose, but if vomiting or diarrhoea continues, seek medical help.

• If a dose is missed, it should be taken as soon as it is remembered.
• The next due dose should be taken at the usual time.
• However, if the missed dose is remembered in close proximity to the next due dose (i.e. within 3 to 4 hours of the next dose), please seek medical advice.
• If unsure about what to do, the treating GP, epilepsy nurse, paediatrician or neurologist should be contacted.

Always read the label. Follow the instructions of your doctor and pharmacist. 

See more about how to store different types of Phenytoin 

Patients of Han Chinese ancestry may have a genetic marker which can increase the risk of severe skin reactions. Please discuss with your doctor and a blood test can be done to screen for this gene before starting this medication. 

Possible side effects include: 

• Loss of appetite, nausea or vomiting 

• Drowsiness or dizziness, feeling nervous or excitable 

• Trouble sleeping 

• Change in taste 

• Blurred vision or double vision – usually if the medication levels are too high 

• Slurred speech, confusion or unsteadiness – usually if the medication levels are too high 

• Excess hair growth – usually with prolonged use 

• Coarsening of facial features – usually with prolonged use 

• Swollen, tender gums – usually with prolonged use.

More medically serious side effects include: 

• Difficulties with balance and coordination – usually with prolonged use 

• Weak bones – usually with prolonged use 

• Suicidal thoughts or actions or other changes in mood or behaviour 

• Heart problems such as slow heart beat – usually when medication given through the vein 

• Increase in the number of seizures – may occur in some types of epilepsy (Eg: Absence, Myoclonic, Dravet Syndrome) 

Seek urgent medical help if an allergic reaction occurs. This includes symptoms such as:

• Red, swollen, blistered or peeling skin 

• Red or irritated eyes

• Sores in the mouth, throat, nose or eyes potentially with a fever 

Pregnancy and contraception 

If a pregnancy is planned, a discussion with your neurologist is recommended. For further information, refer to the reproductive health page.

The information provided in this resource is limited and does not replace the need for a medical consultation. It is very important you speak to your doctor about all aspects of your medication including side effects when medication is first prescribed. Report any concerns to your doctor promptly.