Cannabinoid derivates have been largely used for different medical purpose. In the literature, several methods capable of separating THC and its principles metabolites are described, although Δ8- and Δ9-THC separation has not been completely achieved. THC metabolism has not been fully understood and metabolites plasma distribution in healthy and pathological patients remains to further deepen. The aim of this study was the validation of UHPLC-MS/MS method for the quantification of 10 cannabinoids in human plasma, as important tool for improving clinical efficacy of cannabis administration. Obtained results were in accordance with recommendations of ICH Harmonised Guideline for bioanalytical method validation, showing a good linearity, optimal accuracy as well as satisfactory results in terms of intra-day and inter-day precision and matrix effect. Furthermore, blood sampling study was performed to investigate the better collection method. Optimal separation of Δ-9-tetrahydrocannabinol (Δ9-THC), Δ8-tetrahydrocannabinol (Δ8-THC) was obtained. The present method showed optimal linearity and satisfactory results in terms of specificity and selectivity. Recovery was between 92.0% and 96.5% for all analytes. The matrix-effect showed good performance; no carry over was observed. Cannabinoid metabolites present in higher plasma concentrations were: 11–Hydroxy–Δ9–tetrahydrocannabinol, 11–Nor–9carboxy–Δ9–tetrahydrocannabinol and THC-COOH-glucuronide. Method performance makes it suitable for routine purposes and a potential tool for therapeutic ranges definition. The present work will be used to test several samples in a long-term clinical study, paving the way for further future works.