Cannabis and Cannabinoids, ( D 9-tetrahydrocannabinol),
CBD (cannabidiol) and CBN (cannabinol).

Cannabis contains a number of cannabinoids, of which the most abundant are THC ( D 9-tetrahydrocannabinol), CBD (cannabidiol) and CBN (cannabinol). The active ingredient, which gets you high, is THC. In the plant, CBD is a precursor and CBN a metabolite of THC. As cannabis gets older, THC gradually breaks down to CBN.

THC is the substance found to have most psychoactive effect, and also the most effect on symptoms such as pain or bronchospasm. CBN and CBD are not psychoactive themselves; however CBD can take the edge off a THC high, tending to relax the user rather than increasing anxiety and paranoia. CBD has also been found to have anticonvulsant properties (potential value in treating epilepsy, MS, muscle spasms etc), and recently published research has indicated CBD to be effective in reducing neurotoxic brain damage following strokes or head trauma. The proposed mechanism of action also suggests CBD may offer some protection against ecstasy-derived neurotoxicity (long-term depletion of serotonergic receptors), although this has not specifically been investigated. CBD is normally present in significant quantities in hashish (cannabis resin), although in most herbal cannabis or "skunk" the levels of CBD are usually low or absent.

Although smoked cannabis offers some advantages over oral preparations (rapid onset of action, ability to control dose), the smoke contains carcinogens, tars and other lung irritants which may cause respiratory diseases and lung cancer. The lack of an effective filter in "roaches" and the deep inhalation techniques used by cannabis smokers increases this risk. However the study of smoke composition quoted by the BMA and others referred to poor quality herbal cannabis smoked "neat", and the relevance of these results to tobacco/hash spliffs or high-THC varieties is questionable. At least one pharmaceutical company is developing alternative quick-acting delivery systems for plant-derived cannabinoid combinations, avoiding the need for smoking.