According to research at the California Pacific Medical Center
Research Institute in San Francisco cannabidiol (CBD) increased
the inhibitory effects of dronabinol (THC) on human brain cancer
cell proliferation and survival. The two natural cannabinoids were
tested on two glioblastoma cells lines. THC and CBD acted
synergistically to inhibit cell proliferation. The treatment of
glioblastoma cells with both compounds led to significant
modulations of the cell cycle, induction of reactive oxygen species
(free radicals) and apoptosis (programmed cell death).

There were specific changes that were not observed with either
compound individually, indicating that the signal transduction
pathways affected by the combination treatment were unique.
Researcher concluded that these "results suggest that the addition
of cannabidiol to delta-9-THC may improve the overall
effectiveness of  delta-9-THC in the treatment of glioblastoma in
cancer patients."

(Source: Marcu JP, Christian RT, Lau D, Zielinski AJ, Horowitz
MP, Lee J, Pakdel A, Allison J, Limbad C, Moore DH, Yount GL,
Desprez PY, McAllister SD.Cannabidiol enhances the inhibitory
effects of Delta9-tetrahydrocannabinol on human glioblastoma cell
proliferation and survival. Mol Cancer Ther 2010;9(1):180-9.)