Home› Drug Doses› Phenytoin
⚡ IV phenytoin: MAX 50 mg/min (adult) · 1 mg/kg/min (child). Faster rates cause cardiac arrhythmia, hypotension, and cardiac arrest. ECG and BP monitoring mandatory during loading. Dilute in Normal Saline ONLY — never dextrose.
💊 Anticonvulsant · Narrow therapeutic index · TDM mandatory

Phenytoin Dose Calculator

India · Loading · Maintenance · IBW-Based Dosing · TDM Targets · Albumin-Corrected Level · Eptoin · Dilantin

Load: 15–20 mg/kg IV Maint: 4–8 mg/kg/day TDM: 10–20 mg/L IBW-based dosing

Phenytoin Dose Calculator

Phenytoin Dose
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TDM check
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🧪 TDM Targets
Therapeutic range10–20 mg/L
Target for epilepsy10–20 mg/L (trough)
Toxic level>20 mg/L (nystagmus); >40 mg/L (encephalopathy)
Sample timingTrough (pre-dose) after 5–7 days of stable dosing
Half-lifeHighly variable: 7–42 hours (dose-dependent)
🩺 Albumin-Corrected Phenytoin Level
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Albumin—
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💊 Drug profile
ClassHydantoin anticonvulsant
RoutesOral · IV (slow)
Half-life7–42h (dose-dependent)
Protein binding~90%
TDM target10–20 mg/L (total)
Bioavailability~100% capsule; ~90% tablet
KineticsNon-linear (Michaelis-Menten)
🏷️ Indian brands
Cap 100mgEptoin 100 · Dilantin · Phenytek
Tab 50mg (Infatabs)Eptoin 50 · Dilantin Infatab
Susp 125mg/5mlEptoin susp · Dilantin susp
IV 50mg/mlEptoin IV · Phenytoin Inj (generic)
⚠️ Major drug interactions

🔼 Levels increased by: isoniazid, fluconazole, omeprazole, valproate, cimetidine

🔽 Levels decreased by: carbamazepine, rifampicin, phenobarbitone, St John's Wort

⚠️ Reduces effectiveness of: OCP, warfarin, digoxin, dexamethasone, cyclosporine

Phenytoin — Clinical Guide India

Phenytoin (Eptoin, Dilantin) remains one of the most widely used anticonvulsants in India for generalised tonic-clonic seizures, focal epilepsy, status epilepticus, and post-neurosurgical seizure prophylaxis. It has a narrow therapeutic index and highly non-linear (Michaelis-Menten) pharmacokinetics — small dose increases can cause disproportionately large rises in serum levels. TDM is mandatory for safe phenytoin use, and the albumin correction formula must be applied in any patient with hypoalbuminaemia.

IBW-based dosing — why it matters

For IV loading and initial oral maintenance, phenytoin dose should be calculated on Ideal Body Weight (IBW) rather than actual body weight in obese patients. IBW underestimates the appropriate dose because phenytoin does partly distribute into adipose tissue. Use Adjusted Body Weight (AdjBW) for loading: AdjBW = IBW + 1.33 × (actual − IBW). For maintenance, actual weight-based dosing gives reasonable starting estimates, but TDM is essential for individualisation due to highly variable pharmacokinetics.

IV phenytoin — rate is everything

The maximum IV infusion rate for phenytoin in adults is 50 mg/min. Faster rates cause cardiovascular toxicity (hypotension, bradycardia, ventricular arrhythmia, cardiac arrest) due to the propylene glycol diluent in the formulation. For a typical adult loading dose of 1000–1400 mg: infuse over at least 20–28 minutes. Continuous ECG monitoring and blood pressure monitoring are mandatory during loading. Have resuscitation equipment at the bedside. Dilute in 0.9% normal saline — NOT in dextrose (phenytoin precipitates in glucose solutions).

Albumin-corrected phenytoin — essential in India

Phenytoin is ~90% bound to serum albumin. In patients with hypoalbuminaemia (malnutrition, liver disease, nephrotic syndrome, critical illness — all common in India), the measured total phenytoin level underestimates pharmacologically active free drug. The Winter-Tozer formula corrects for this: Corrected phenytoin = Measured level ÷ [0.9 × (albumin/4.4) + 0.1]. For patients with severe CKD (CrCl <10 mL/min): Corrected = Measured ÷ [0.1 × (albumin/4.4) + 0.1]. Target: corrected level 10–20 mg/L. A patient with measured level 6 mg/L but albumin of 2 g/dL actually has a corrected level of approximately 12 mg/L — within therapeutic range despite appearing subtherapeutic.

Frequently Asked Questions

What is the phenytoin IV loading dose for a 70 kg adult?+
Standard phenytoin IV loading dose: 15–20 mg/kg. For a 70 kg adult: 1050–1400 mg IV. Use 15 mg/kg (1050 mg) for seizure prevention; 20 mg/kg (1400 mg) for active status epilepticus. Infuse at maximum 50 mg/min → minimum 21–28 minutes infusion time. Dilute in 100–250 ml of 0.9% normal saline. Monitor ECG and BP throughout.
Why does phenytoin have non-linear pharmacokinetics?+
Phenytoin undergoes saturable (Michaelis-Menten) hepatic metabolism — at low doses, the metabolising enzyme (CYP2C9/2C19) is not saturated and phenytoin behaves linearly. As doses approach saturation, small dose increases cause disproportionately large rises in plasma levels. This is why dose adjustments must be made in small increments (25–50 mg at a time for adults) and TDM is essential. A dose increase from 300mg to 350mg/day may raise levels from 10 to 20 mg/L.
What are the signs of phenytoin toxicity?+
Phenytoin toxicity is dose-related: nystagmus (level >20 mg/L), ataxia and gait instability (>25 mg/L), diplopia and dysarthria (>30 mg/L), drowsiness and confusion (>35 mg/L), severe encephalopathy (>40 mg/L). Chronic toxicity causes peripheral neuropathy, gingival hyperplasia, hirsutism, and folate deficiency. If toxicity is suspected, hold phenytoin and check a trough level immediately.
⚠️Phenytoin has a narrow therapeutic index and non-linear kinetics. IV infusion must never exceed 50 mg/min. Always apply albumin correction in hypoalbuminaemia. TDM is mandatory. Verify against BNF, BNFC, and current prescribing information.

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