Frequent plasma sampling to monitor pharmacokinetic (PK) profile of antiepileptic drugs (AEDs), is invasive, costly and time consuming. For drugs with a well-defined PK profile, such as AED lacosamide, equations can accurately approximate PK parameters from one steady-state plasma sample.Equations were derived to approximate steady-state peak and trough lacosamide plasma concentrations (C peak,ss and C trough,ss , respectively) and area under concentration–time curve during dosing interval (AUC τ,ss ) from one plasma sample. Lacosamide (k a : ∼2h −1 ; k e : ∼0.05h −1 , corresponding to half-life of 13h) was calculated to reach C peak,ss after ∼1h (t max,ss ). Equations were validated by comparing approximations to reference PK parameters obtained from single plasma samples drawn 3–12h following lacosamide administration, using data from double-blind, placebo-controlled, parallel-group PK study. Values of relative bias (accuracy) between −15% and +15%, and root mean square error (RMSE) values ≤15% (precision) were considered acceptable for validation.Thirty-five healthy subjects (12 young males; 11 elderly males, 12 elderly females) received lacosamide 100mg/day for 4.5 days. Equation-derived PK values were compared to reference mean C peak,ss , C trough,ss and AUC τ,ss values. Equation-derived PK data had a precision of 6.2% and accuracy of −8.0%, 2.9%, and −0.11%, respectively. Equation-derived versus reference PK values for individual samples obtained 3–12h after lacosamide administration showed correlation (R 2 ) range of 0.88–0.97 for AUC τ,ss . Correlation range for C peak,ss and C trough,ss was 0.65–0.87. Error analyses for individual sample comparisons were independent of time.Derived equations approximated lacosamide C peak,ss , C trough,ss and AUC τ,ss using one steady-state plasma sample within validation range. Approximated PK parameters were within accepted validation criteria when compared to reference PK values.