To investigate what the body does to single doses of Sativex (i.e. the pharmacokinetics [PKs] of four sprays containing 10.8 mg Δ9 tetrahydrocannabinol [THC] and 10 mg cannabidiol [CBD]) when mild, moderate or severe oral mucositis is induced. This will be done by looking at the effects of the body on the drug before and after oral mucositis is induced. The study participants will have Non-surgical Head and Neck Squamous Cell Carcinoma (HNSCC), and oral mucositis will be induced with radiotherapy and/or chemotherapy.