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6. The chronic effects of cannabis use on health
Cellular and immunological effects
The possible effects of chronic cannabis use on cellular processes and the immune system are considered together because both effects may influence a cannabis user's susceptibility to diseases. If cannabis use affects cellular processes then users may be at increased risk of developing various types of cancer, and if it affects the immune system then cannabis users may be at increased risk of contracting infectious diseases and developing cancer.
6.1 Mutagenicity and carcinogenicity
A major reason for research into the effects of cannabinoids on cellular processes is to discover whether cannabinoids are mutagenic, i.e. whether they may produce mutations in the genetic material in the somatic and germ cells of users. If cannabinoid exposure affects the genetic material of a user's somatic or bodily cells (such as those of the lung, for example) then chronic cannabis use may cause cancer. If it affects the genetic material of germ cells (the sperm and ova), then genetic mutations could be transmitted to the children of cannabis users.
There is experimental evidence from in vitro studies of animal cells that some cannabinoids, including THC, can produce a variety of changes in cellular processes in vitro (i.e. in the test tube). These include alterations to cell metabolism, DNA synthesis, and cell division (Nahas, 1984). The potential for cannabinoids to produce genetic change in humans or animals is unclear. There is, at most, mixed evidence that THC and other cannabinoids are mutagenic in standard microbial assays, such as the Ames test, and there is contradictory evidence on whether the cannabinoids are clastogenic, i.e. produce breaks in chromosomes. According to Bloch (1983) who reviewed the literature for the World Health Organisation: "in vivo and in vitro exposure to purified cannabinoids or cannabis resin failed to increase the frequency of chromosomal damage or mutagenesis" (p412). Nahas (1984) reviewed the same evidence and concluded that "cannabinoids and marihuana may exert a weak mutagenic effect" (p117). More recently, Zimmerman and Zimmerman (1990/1991) concluded that "cannabis mutagenicity remains unclear", but argued that there was evidence that "cannabinoids induce chromosome aberrations in both in vivo and in vitro studies" (p19).
There is stronger and more consistent evidence that cannabis smoke, like smoke produced by most burning plant material, is mutagenic in vitro, and hence, is potentially carcinogenic (Leuchtenberger, 1983). According to Bloch (1983) "marijuana smoke exposure has been reported to be associated with chromosomal aberrations ... [such as] hypoploidy, mutagenicity in the Ames test ... " (Bloch, 1983, p413). This is consistent with research indicating that cannabis smoke contains many of the same carcinogens as cigarette smoke (Institute of Medicine, 1982; Leuchtenberger, 1983), suggesting that if cannabis smoke is carcinogenic it is more likely to be because of the carcinogens it shares with cigarette smoke rather than because of the cannabinoids it contains. If it is the non-cannabinoid components of cannabis smoke that are mutagenic, then any cancers caused by cannabis smoking are most likely to develop after long-term exposure to cannabis smoke, and they are most likely to develop at sites which have had the maximum exposure to that smoke, namely, the upper aerodigestive tract and lung. This possibility is considered in more detail below (see pp49-50).