Phenytoin Dosing Calculator

The Phenytoin Dosing Calculator provides corrected phenytoin level estimation and loading dose calculations for this widely used antiepileptic drug. Phenytoin (Dilantin) is a first-generation antiepileptic drug that remains a mainstay for the treatment of focal and generalized tonic-clonic seizures, status epilepticus, and certain neuropathic pain conditions. Its complex pharmacokinetics — including saturable (Michaelis-Menten) metabolism, high protein binding, and numerous drug interactions — make it one of the most challenging drugs to dose and monitor in clinical practice.

Phenytoin is approximately 90% bound to serum albumin in patients with normal albumin levels and renal function. Only the unbound (free) fraction is pharmacologically active. In hypoalbuminemic states or renal impairment, a greater fraction of phenytoin is unbound, meaning that the measured total level underestimates the active drug concentration. The Winter-Tozer equation corrects for this: Corrected Level = Measured Level / (0.2 × Albumin + 0.1) for normal renal function, or Corrected Level = Measured Level / (0.48 × Albumin + 0.1) for renal impairment/dialysis.

The therapeutic range for phenytoin is 10-20 mcg/mL (total) or 1-2 mcg/mL (free). For patients with altered protein binding, the corrected total level or direct free level measurement provides more accurate therapeutic assessment. Free phenytoin levels, when available, are preferred over calculated corrections, as the Winter-Tozer equation has known limitations and may not be accurate in all clinical scenarios.

Phenytoin exhibits Michaelis-Menten (saturable, zero-order) kinetics at therapeutic concentrations, meaning that small dose changes can produce disproportionately large changes in serum levels. This nonlinear relationship between dose and level distinguishes phenytoin from most other drugs and is the primary reason for its dosing complexity. Near the saturation point (Km), even a 10% dose increase can cause a >50% rise in serum level.

Loading doses are used when rapid therapeutic levels are needed, such as in status epilepticus or new-onset seizures requiring urgent treatment. The standard IV loading dose is 15-20 mg/kg phenytoin equivalents, administered at a maximum rate of 50 mg/min (or 150 mg PE/min for fosphenytoin) to minimize the risk of hypotension and cardiac arrhythmias. Oral loading can use 400 mg, then 300 mg at 2 and 4 hours (total 1 gram over 4 hours for a 70 kg patient).

This calculator implements the Winter-Tozer correction for albumin and renal function, estimates supplemental loading doses when levels are subtherapeutic, and provides therapeutic status assessment. It accounts for the volume of distribution (approximately 0.7 L/kg) and oral bioavailability (approximately 92% for extended-release capsules). All loading doses are rounded to practical increments and capped at safe maximums.

Numerous drug interactions affect phenytoin levels. Strong enzyme inducers (carbamazepine, rifampin) decrease levels, while inhibitors (fluconazole, amiodarone, isoniazid, valproic acid) increase levels. Valproic acid has a dual interaction: it displaces phenytoin from albumin (increasing free fraction) while inhibiting metabolism (increasing total levels). In these complex interactions, free phenytoin levels provide the most reliable monitoring parameter.

This calculator supports clinical decision-making for phenytoin dosing, albumin correction, and loading dose estimation in clinical practice.