Toxicological properties of Δ9-tetrahydrocannabinol and cannabidiol

Cannabis sativa L. contains more than 100 phytocannabinoids that can interact with cannabinoid receptors CB1 and CB2. None of the cannabinoid receptor ligands is entirely CB1- or CB2-specific. The effects of cannabinoids therefore differ not just because of different potency at cannabinoid receptors but also because they can interact with other non-CB1 and non-CB2 targets, such as TRPV1, GPR55, and GPR119. The most studied phytocannabinoid is Δ9-tetrahydrocannabinol (THC). THC is a partial agonist at both cannabinoid receptors, but its psychotomimetic effect is produced primarily via activation of the CB1 receptor, which is strongly expressed in the central nervous system, with the noteworthy exception of the brain stem. Although acute cognitive and other effects of THC are well known, the risk of irreversible neuropsychological effects of THC needs further research to elucidate the association. Unlike THC, phytocannabinoid cannabidiol (CBD) does not appear to have psychotomimetic effects but may interact with some of the effects of THC if taken concomitantly. CBD administered orally has recently undergone well-controlled clinical trials to assess its safety in the treatment of paediatric epilepsy syndromes. Their findings point to increased transaminase levels as a safety issue that calls for postmarketing surveillance for liver toxicity. The aim of this review is to summarise what is known about acute and chronic toxicological effects of both compounds and address the gaps in knowledge about the safety of exogenous cannabinoids that are still open.