In this report we describe the procedures for biobank sample analysis of human breast milk and plasma samples of cetirizine, venlafaxine, metformin, and prednisolone, as well as the active metabolite of venlafaxine; O-desmethylvenlafaxine from lactation studies from the ConcePTION project. The methods were developed and validated for analysis on a UHPLC-MS/MS system with a rapid chromatographic gradient allowing for the analysis of all studied compounds within 3 minutes per run. We validated stability, dilution integrity, accuracy and precision of the method, calibration curve range, selectivity and specificity, determination of lower limit of quantification (LLOQ) in the relevant matricies. The validation confirmed that all parameters were within the acceptance criteria described in the regulatory guidelines and that the stability of the compounds in human whole blood, plasma and breast milk was sufficient for the logistics of the demonstration studies, allowing for the processing and handling of the samples prior to analysis. The validated methods were applied to the analysis of samples from the performed lactation studies, successfully quantifying the compounds in the clinical samples and describing the pharmacokinetics of cetirizine in human breast milk. The data from the cetirizine study was used to develop a population pharmacokinetic model of cetirizine in human breast milk and estimating relative infant dose.