Drug formulation comprises a parent substance and at least its metabolites. An independent claim is also included for preparing a formulation, comprising (a) providing an organism, whose genotype or phenotype, a mother substance and at least a metabolites of the parent compound, an optimal reference plasma levels is at steady state for the parent compound for a reference genotype or reference phenotype upon submission of the parent compound alone in an input module, (b) transferring data from (a) into a calculation module comprising a substance data module, an organism data module, a genotype data module or phenotype data module, and a physiology-based pharmacokinetic model, where the substance data module has data of the physicochemical and/or biochemical properties of the substance, the organism module has data on the compartments of the organism and the genotype data module or phenotype data module have genotype or phenotype-specific data, (c) automatically selecting parent substance and metabolite-specific data from a substance data module, (d) automatically selecting organism-specific data from the organism data module from the input of (a), (e) automatically selecting the genotype-specific or phenotype-specific data from a genotype data module or phenotype data module, (f) transferring the data from (a)-(e) into the physiology-based pharmacokinetic model, (g) calculating an optimized dose of the parent substance by the physiology-based pharmacokinetic model for the reference genotype or reference phenotype for achieving the optimum input reference plasma levels for the parent substance of (a), (h) calculating the reference plasma levels at steady-state of the metabolites (N-desmethyltamoxifen, 4-hydroxytamoxifen, endoxifen) for the reference genotype or reference phenotype during administering the parent compound in calculated dose, (i) calculating reduced plasma levels of the metabolites based on the genotype and phenotype compared to the corresponding refere
