Cannabinoïds and cancer: causation, remediation, and palliation
Wayne Hall, MacDonald Christie, David Currow
Lancet Oncology, 2005, 6, 35–42
This review discusses three different associations between cannabinoids and cancer. First, it assesses evidence that smoking of cannabis preparations may cause cancers of the aerodigestive and respiratory system. There have been case reports of upper-respiratory-tract cancers in young adults who smoke cannabis, but evidence from a few epidemiological cohort studies and case-control studies is inconsistent. Second, there is mixed evidence on the effects of THC and other cannabinoids on cancers: in some in vitro and in vivo studies THC and some synthetic cannabinoids have had antineoplastic effects, but in other studies THC seems to impair the immune response to cancer. As yet there is no evidence that THC or other cannabinoids have anticancer effects in humans. Third, 9-tetrahydro-cannabinol (THC) may treat the symptoms and side-effects of cancer, and there is evidence that it and other cannabinoids may be useful adjuvant treatments that improve appetite, reduce nausea and vomiting, and alleviate moderate neuropathic pain in patients with cancer. The main challenge for the medical use of cannabinoids is the development of safe and effective methods of use that lead to therapeutic effects but that avoid adverse psychoactive effects. Furthermore, medical, legal, and regulatory obstacles hinder the smoking of cannabis for medical purposes. These very different uses of cannabinoids are in danger of being confused in public debate, especially in the USA where some advocates for the medical use of cannabinoids have argued for smoked cannabis rather than pharmaceutical cannabinoids. We review the available evidence on these three issues and consider their implications for policy.
Illicit drug users obtain cannabis preparations from the Cannabis sativa plant, which contains more than 60 cannabinoids.1 The cannabinoid 9-tetrahydrocannabinol (THC)2,3 causes many of the plant’s
psychoactive effects and is found in the resin that covers the flowering tops and upper leaves of the
female plant (figure 1); by contrast, marijuana is prepared from the dried flowering tops and leaves of
the female plant.4 The flowering tops have the highest THC concentration, and concentrations are much
lower in the leaves, stems, and seeds. Cannabis, which normally grows male and female flowers on separate plants, can be cultivated to maximise its THC content by the sinsemilla method, in which female plants are isolated, causing them to increase production of flowers covered by resin glands containing THC or other cannabinoids.4
The concentration of THC (figure 2) in marijuana has been reported to range from 0·5% to 5·0%,5 whereas sinsemilla cannabis may contain 7·0–14·0% THC.5 Hashish, or hash, consists of dried cannabis resin that contains 2·0–8·0% THC. Hash oil is made by extraction of THC from hash (or marijuana) in oil and contains 15·0–20·0% THC.6 Evidence suggests that cannabis in the Netherlands may contain much higher amounts of THC than previously thought.7 More accurate data on the THC content of cannabis products is needed, as is research on the health implications of increased THC content.7
In this discussion, cannabis refers to products derived from C sativa that are assumed to be smoked, unless otherwise specified. Cannabinoids refer to pharmaceutical extracts derived from C sativa and to synthetic substances that act on cannabinoid receptors in the brain. Endocannabinoids refer to endogenous cannabinoids that are found in the human brain and body.