Phenytoin

Question 1

What is the mechanism of action of phenytoin and what is it used for

Answer 1

Inhibition of voltage-gated sodium channels to prevent the repetitive firing of action potentials

Indication: epilepsy

Question 2

What is the therapeutic range of phenytoin

Answer 2

10 to 20mg/L

Question 3

What is the dose equivalence of phenytoin sodium vs base

Answer 3

Phenytoin sodium = 100mg

Phenytoin base= 92mg

Question 4

What are the warning signs of phenytoin that require referral to the doctors

Answer 4

• Toxicity: nystagmus, vision doubles, slurred speech, ataxia, hyperglycaemia
• Skin disorder: rash, toxic epidermal necrolysis
• Hepatotoxicity: jaundice, GI pain, dark urine
• Blood disorders: bleeding, bruising, fever, mouth ulcers, sore throat
• Suicidal thoughts
• LOW VITAMIN D LEVELS- rickets in childeren, osteomalacia in adults

Question 5

What are the drug interactions in phenytoin

Answer 5

Increased plasma concentration with: 
Amiodarone
DISULFIRAM 
DILTIAZEM 
FLUCONAZOLE
FLUOXETINE
MICONAZOLE
CLARITHROMYCIN

Reduced plasma concentration with: 
St Johns wort 
Theophylline
Ciclosporin
ITRACONAZOLE