A 3-year-old Leonberger was presented with unilateral atrophy of the left temporal muscle, concomitant left corneal hypoesthesia and jaw pain. Based on magnetic resonance imaging (MRI) results, a trigeminal peripheral nerve sheath tumor was presumptively diagnosed, and treatment based on volumetric modulated arc therapy combined with temozolomide was introduced. Although the tumor volume was stable, after 6 months the patient developed severe jaw pain manifested by rubbing of the face, aversion to eating, and yawning. No desirable response to combinations of meloxicam, gabapentin, tramadol/buprenorphine at their maximal tolerated doses was achieved. Only a treatment based on a commercially available nonpsychoactive cannabinoid oil – Perognidol® (Ilovet) combined with gabapentin successfully relieved the symptoms of pain within a week. Three months later, a follow-up MRI revealed a significant growth of the tumor associated with cerebral infiltration. Eventually, the general condition of the patient deteriorated. The dog was euthanized 800 days after the original diagnosis.