Cannabis is the most widely used illicit drug in the world. Cannabinoids are used therapeutically by some patients as they have analgesic, anti-emetic and appetite stimulant properties which palliate adverse symptoms. Use of these agents in an oncology setting raises the question of whether they act to modulate the effectiveness of concurrently administered anti-cancer drugs. The transporter, P-glycoprotein (P-gp) confers multiple drug resistance (MDR) by effluxing a diverse array of anti-cancer agents. This study was undertaken to examine the effect of cannabinoids on P-gp. Unlike the known P-gp inhibitor, PSC833, short 1h exposure to three plant-derived cannabinoids, cannabinol (CBN), cannabidiol (CBD) and Delta(9)-tetrahydrocannabinol (THC) and the synthetic cannabinoid receptor agonist, WIN55, 212-2 (WIN) did not inhibit the efflux of the P-gp substrate Rhodamine 123 (Rh123) in either a drug-selected human T lymphoblastoid leukaemia cell line (CEM/VLB(100)) or in a mouse fibroblast MDR1 transfected cell line (77.1). However, in CEM/VLB(100) cells, prolonged 72 h exposure to the cannabinoids, THC and CBD, decreased P-gp expression to a similar extent as the flavonoid, curcumin (turmeric). This correlated with an increase in intracellular accumulation of Rh123 and enhanced sensitivity of the cells to the cytotoxic actions of the P-gp substrate, vinblastine. Taken together, these results provide preliminary evidence that cannabinoids do not exacerbate P-gp mediated MDR. Further, plant-derived cannabinoids are moderately effective in reversing MDR in CEM/VLB(100) cells by decreasing P-gp expression.