The active constituents of cannabis (predominantly cannabinoids and possibly flavonoids) are more effective than a single cannabinoid in reducing muscle spasticity in a multiple sclerosis animal model. Possible interactions include both pharmacokinetic and pharmacodynamic effects. Synthetic cannabinoids with agonist or antagonist/inverse agonist effects are available with high affinity for cannabinoids receptors. Pharmaceutical products that block their uptake and metabolism may enhance the endocannabinoid system. Government- and charity-funded clinical trials of cannabis are proceeding in pain, both acute (MRC multicenter trial) and chronic, and multiple sclerosis (MRC and Multiple Sclerosis Society multicenter trials). The design of these trials should enable evidence to be presented to regulatory bodies documenting the medicinal uses of standardized cannabis plant material.