The increasing demand for Cannabis-derived medicinal products in Brazil, coupled with the implementation of RDC 1013/2026 (ANVISA), necessitates robust analytical frameworks for pharmacological and forensic purposes. This study is part of the "Paracelsus Project," a feasibility study evaluating the production of plant-based active pharmaceutical ingredients (API) from seized Cannabis sativa L. (marijuana), aiming to repurpose material that is currently incinerated. Preliminary data suggests this seized material contained a total of roughly 25 tons of CBD. A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed and validated for the simultaneous determination of twelve phytocannabinoids (CBD, D9-THC, D8-THC, CBC, CBL, CBN, CBG, THCV, CBDV, CBDVA, CBGA, and THCA). The methodology utilized a triple quadrupole (QQQ) analyzer in positive ionization mode, designed to overcome the challenges of complex plant matrices and chemical isomers. The method achieved successful baseline separation of critical pairs, such as CBD/CBG and D9-THC/D8-THC, in under eight minutes. This analytical workflow was successfully applied to monitor cannabinoid concentrations during the extraction and purification processes from seized marijuana by Brazilian Federal Police. The validated LC-MS/MS method provides the necessary sensitivity and specificity for both quality control in API production and forensic standardization. By establishing a technical foundation for characterizing medicinal extracts and ensuring compliance with the 0.3% THC statutory threshold, this research supports public health safety and the potential for a sustainable pharmaceutical supply chain from seized materials in Brazil.