Abstract: A method for determining an effect of a drug on a patient, with at least one specific dose regimen. A population pharmacological model describing a variability in drug exposure within a patient population receiving known doses of the drug is obtained. For at least one parameter relating to a source of the variability within the patient population, a plurality of values of the parameter is sampled from a distribution conditional on individual patient characteristics and/or at least one measurement related to a drug exposure of the at least one patient. By using the population pharmacological model, a corresponding drug exposure with given dose regimens is calculated, for sets of the sampled simulated values. The effect of the drug on a given patient, with at least one specific dose regimen, is determined based at least on the calculated drug exposures. Also, a corresponding computer program and processing circuit.