High-grade glioma is one of the most aggressive cancers in adult humans and long-term survival rates are very low as standard treatments for glioma remain largely unsuccessful. Cannabinoids have been shown to specifically inhibit glioma growth as well as neutralize oncogenic processes such as angiogenesis. In an attempt to improve treatment outcome, we have investigated the effect of D9 -tetrahydrocannabinol (THC) and cannabidiol (CBD) both alone and in combination with radiotherapy in a number of glioma cell lines (T98G, U87MG, and GL261). Cannabinoids were used in two forms, pure (P) and as a botanical drug substance (BDS). Results demonstrated a duration- and dose-dependent reduction...