Drug inventory management has a great importance for all members within the pharmaceutical supply-chain due to the important role that drugs play to save the lives of patients and the need for their availability on time in the necessary amount. To provide the necessary quantity of drugs at the right moment and satisfy the patient’s needs, there are many methods, like the coordination between different members of the pharmaceutical supply chain. In fact, this paper proposes a collaborative multi-echelon pharmaceutical supply chain composed of a central pharmacy, multi-regional pharmacies and multi-hospitals using a stochastic multi-period and multi-product mathematical model for the minimisation of the drug-inventory-management costs, such as shortage and holding costs. On the other hand, the stochastic mathematical model is solved using the Lingo 18.0 software and brunch and bound method. Subsequently, a comparison between this model and the current situation of the Tunisian pharmaceutical supply chain was-made to highlight the benefits of this research work.