PHENYTOIN
Available Dosage Forms
Form
Strength
Chemical Name
Phenytoin Sodium
Phenytoin Acid
Parenteral
50 mg/ml
Generic available
Immediate
Absorption Capsule
Extended
Absorption
Capsule*
30 mg, 100 mg
Generic available
30 mg, 100 mg
200 mg, 300 mg
Dilantin Kapseals
Phenytek
50 mg
30 mg/ml
125 mg/ml
100 mg/4 ml
Dilantin Infatabs
Dilantin-30
Dilantin-125
Xactdose
Chewable Tablets
Oral Suspension
Fosphenytoin
Brand Name
Parenteral
150 mg
Cerebyx
750 mg
*Capsule may be opened and mixed with applesauce or pudding. DO NOT crush or dissolve
capsule granules.
Dosing
Loading Dose (IV/PO*)
Maintenance Dose (IV/PO)
“Mini Load” (IV/PO)**
10-20 mg/kg (15 mg/kg)
4-7 mg/kg/day (5 mg/kg)
(Desired level – actual level) x weight
(TBW)
*Split total PO dose into smaller doses and separate administration by 2 hrs
**AFTER TDM levels have returned and patient is subtherapeutic
Phenytoin Equivalents (PE)
150 mg Fosphenytoin = 100 mg Phenytoin Sodium
1.5 mg Fosphenytoin = 1 PE
Maximum Infusion Rate
Phenytoin
Fosphenytoin
50 mg/minute
150 mg/minute
Maximum PO Dose
300-400 mg as one time dose*
*Due to maximum absorption
Therapeutic Drug Levels and Monitoring
Corrected Phenytoin Level*
Therapeutic Level Range*
10-20 mcg/ml
Measured Phenytoin level / ((0.2 x Measured Albumin) + 0.1)
*Correct if albumin< 4 g/dl
After IV Load Monitoring
2-4 hours after loading dose
After PO Load Monitoring
6-8 hours after final loading dose
Maintenance Monitoring (IV/PO)
5-7 days (during hospitalization)
*Correct for albumin <4 g/dl
Level Associated Adverse Drug
Reactions/Toxicities
>20 mcg/ml
Nystagmus
>30 mcg/ml
Ataxia, seizures
>40 mcg/ml
Somnolence/Coma
Common Adverse Events
Cardiac arrhythmia,
Tremor or slurred speech
cardiovascular collapse,
hypotension*
Hirsutism, rash
Gum hypertrophy
Decreased cognitive
Respiratory and
ability
circulatory depression
*Occurs with RAPID IV infusion (>50 mg/min)
Common Drug Interactions*
Ritonavir
Antacids
Warfarin
Ciprofloxacin
tube feeds**
Digoxin
Valproic acid
HMG-CoA Reductase Inhibitors
(“Statins”)
SSRIs
Azole Antifungals
*Phenytoin, is a potent inducer and substrate of CYP-450 enzymes (3A4, 2C9,
2C19), consult drug interaction database
**Stop tube feeds 2 hours before administration of phenytoin and restart 2 hours
after dose