Substance Use Disorder (SUD) represents a global health challenge. Digital therapeutics, particularly those based on Virtual Reality (VR), offer a promising paradigm for increasing the efficacy and accessibility of treatment by providing immersive, controlled environments. This thesis documents the design and implementation of a standalone VR application for Meta Quest, developed in Unity, intended to support SUD therapy. The core of the system is a virtual therapist guided by an advanced conversational agent (Large Language Model, LLM) hosted on a high-performance computing (HPC) infrastructure at the Politecnico di Torino. The therapeutic flow is structured into sequential modules. The patient, guided by the virtual therapist, completes a pre-assessment analysis via an in-VR tablet interface to record levels of craving, pain, and mood.