Optimizing a Phenytoin Dosage Regimen

At the clinic you have a new patient (D.S.), age 57 years and weight 90 kg. An initial dose of 495 mg/day was recommended. The patient was instructed to take this dose each day for a few weeks and return for a plasma sample. This plasma sample was found to contain 7.3 mg/L phenytoin. This was somewhat low so you need to calculate a new dose per day with the aim of achieving an average plasma concentration of 15 mg/L. For this calculation you can assume that the patient has a Km value of 4 mg/L (a population average value).

A useful equation

Equation 31.1.1 Dose Rate equals Rate of Elimination

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