Users

Phenytoin Monitoring

Affected Drug

• N03AB02 Phenytoin

Baseline

Baseline: CBC, ALT/AST

Follow-up

Follow-up: CBC, ALT/AST annually. Folate if anemic Serum Levels: Measure trough (30 min prior to next dose) phenytoin level under the following circumstances** 1. 2 weeks following initiation and q 2 weeks until target level reached (40-80μmol/L) and seizure free. 2. 2 weeks following initiation of interacting drug 3. At the time of breakthrough seizures 4. Suspected dose-related phenytoin toxicity ** Albumin must be measured with serum phenytoin (except with free phenytoin) for correction ** Corrected phenytoin level = observed level/((0.025 x alb)+0.1) ** Dose changes should be small and generally should not exceed 50mg per change

Important Interactions

• Drugs: C01BD01 Amiodarone
  Interaction ↑ phenytoin: measure level 2 weeks following initiation and adjust if necessary
• Drugs: A02BA01 Cimetidine
  Interaction ↑ phenytoin: measure level 2 weeks following initiation and adjust if necessary
• Drugs: J01MA02 Ciprofloxacin
  Interaction ↑ phenytoin: measure level 2 weeks following initiation and adjust if necessary
• Drugs: J02AC01 Fluconazole
  Interaction ↑ phenytoin: measure level 2 weeks following initiation and adjust if necessary
• Drugs: J04AC01 Isoniazid
  Interaction ↑ phenytoin: measure level 2 weeks following initiation and adjust if necessary.
• Drugs: felbamate, oxcarbazine
  Interaction ↑ phenytoin: measure level 2 weeks following initiation and adjust if necessary.
• Drugs: SSRI
  Interaction ↑ phenytoin: measure level 2 weeks following initiation and adjust if necessary.
• Drugs: TCA
  Interaction ↑ phenytoin: measure level 2 weeks following initiation and adjust if necessary.
• Drugs: topiramate
  Interaction ↑ phenytoin: measure level 2 weeks following initiation and adjust if necessary.
• Drugs: valproic acid
  Interaction can inc or dec phenytoin: measure level 2 weeks following initiation and adjust if necessary.
• Drugs: N03AF01 Carbamazepine
  Interaction ↓ phenytoin: measure level 2 weeks following initiation and adjust if necessary.

Details

Conditions	System	Monitoring		ADE	Incidence	Recommendation
		Baseline	Follow-up
	CNS	none	Physical Exam	drowsiness, dizziness, tremor, ataxia, nystagmus	Common	Consider dose ↓
	HEENT	none	Physical Exam	gingival hyperplasia	50%	Good oral hygiene
	cardiac	BP, HR during infusion of loading dose		hypotension, arrhythmia		↓ infusion rate
	Hepatic	AST/ALT	AST/ALT yearly or if rash occurs	hepatitis	rare	Consider ↓ †“dose or D/C
	Hematologic	CBC	CBC yearly or if rash occurs	leucopenia, agranulocytosis, folate deficiency		Consider ↓ †“dose or D/C, add folic acid
	General	none	Physical exam	rash (rarely severe)	5-10 %	May be anticonvulsant hypersensitivity syndrome (AHS) (higher suspicion if fever present), D/C and investigate for AHS, cross sensitivity to carbamazepine, phenobarbital, lamotrigine

References

• Raebel MA et al. Randomized trial to improve laboratory safety monitoring of ongoing drug therapy in ambulatory patients. Pharmacotherapy. 2006 May;26(5):619-26.; Patsalos PN et al. Antiepileptic drugs - best practice guidelines for therapeutic drug monitoring: a position paper by the subcommission on therapeutic drug monitoring, ILAE Commission on Therapeutic Strategies. Epilepsia. 2008 Jul;49(7):1239-76.; Bohan KH et al. Anticonvulsant hypersensitivity syndrome: implications for pharmaceutical care. Pharmacotherapy. 2007;27(10):1425-39.; Jensen B & Regier L, editors. RxFiles Drug Comparison Charts, 6th edition. RxFiles Academic Detailing Program. 2007.
• Patsalos PN et al. Antiepileptic drugs - best practice guidelines for therapeutic drug monitoring: a position paper by the subcommission on therapeutic drug monitoring, ILAE Commission on Therapeutic Strategies. Epilepsia. 2008 Jul;49(7):1239-76
• Bohan KH et al. Anticonvulsant hypersensitivity syndrome: implications for pharmaceutical care. Pharmacotherapy. 2007;27(10):1425-39
• Jensen B & Regier L, editors. RxFiles Drug Comparison Charts, 6th edition. RxFiles Academic Detailing Program. 2007.