If the patient has not already received phenytoin then give:
Phenytoin sodium IV 18mg/kg (see Table 1 below). Ensure ECG, blood pressure and respiratory function are monitored throughout the duration of the infusion.
|Weight (kg)||IV Loading Dose (mg)||Volume of IV Phenytoin (ml) (vial = 250mg/5ml)|
If phenytoin is already present but the patient is still not controlled, a 'top-up' loading dose may be useful.
Phenytoin sodium 'top-up' dose (mg) = (20 - measured concentration (mg/L)) x 0.7 x weight (kg)
Table 2 gives the approximate increase in concentration following doses of 250–750mg. For example, if the patient weighs 70kg and has a measured concentration of 5mg/L, a single dose of 750mg will increase the concentration to around 20mg/L (5mg/L + 15mg/L).
|Concentration increase with 'top-up' dose|
|Dose / Weight||50 kg||60 kg||70 kg||80 kg|
|250 mg||7 mg/L||6 mg/L||5 mg/L||4.5 mg/L|
|500 mg||14 mg/L||12 mg/L||10 mg/L||9 mg/L|
|750 mg||21 mg/L||18 mg/L||15 mg/L||13.5 mg/L|
Typical phenytoin doses are 3–5mg/kg/day. The first dose should be given 12–24 hours after the loading dose.
Oral or nasogastric administration should be used, whenever possible. Only use intravenous administration when these options are not feasible and where cardiac monitoring is available.
N.B. Table 3 below is for maintenance dose adjustment only. For 'top-up' doses in urgent situations see Table 2 above.
|Measured concentration||Current dose||Maximum dose increase|
Target concentration range: 10–20mg/L
N.B. A plasma level of 5–10mg/L may be adequate for some patients.
Phenytoin is highly protein-bound but only the unbound concentration is active. In patients with low serum albumin concentrations, a higher proportion of the total (measured) phenytoin concentration is unbound and caution is therefore required when interpreting the result.
The equation below gives an albumin corrected total phenytoin concentration, which can be compared with the target concentration range (10–20mg/L).
|Corrected phenytoin concentration =||Measured phenytoin concentration|
|(0.9 x Albumin (g/L) / 42*) + 0.1|
*Midpoint of reference range for serum albumin
N.B. This equation only gives a rough estimate and the patient's clinical condition should be the most important consideration. Seek advice from neurology or pharmacy if you are unsure what to do.
Guideline reviewed: August 2023
Page last updated: October 2023