Carcinogenicity
Zopiclone may be carcinogenic and mutagenic according to rat, mice, and hamster studies. At 100 mg per kg of bodyweight per day, the experimental dosage was considerably higher than the therapeutic dose for humans. The authors of an uncontrolled study of zopiclone said that it may take decades in immunocompetent people before carcinogenic effects from past zopiclone use develops. It was suggested that further research and monitoring was required into the potential for zopiclone to cause cancer in immunocompetent patients.
A recent analysis of both U.S. Food and Drug Administration (FDA) data and clinical trial data shows that nonbenzodiazepine Z-drugs at prescribed doses cause an increased risk of developing cancer in humans. There have been 15 epidemiological studies which have shown that hypnotic drugs cause increased mortality, mainly due to increased cancer deaths. The cancers included those of the brain, lung, bowel, breast, and bladder. One possible explanation for the increased cancer deaths is that the Z-drugs have an adverse effect on the immune system. The fact that clinical trial subjects taking other Z-drugs (zolpidem, zaleplon and eszopiclone) had an increased rate of infections seems to support this theory. Benzodiazepine hypnotic agents are also associated with an increased risk of cancer in humans, namely ovarian cancer. Development of malignancy has been associated with zolpidem usage, but the incidence of neoplasm in zolpidem users is as yet unknown.
Indiplon, another nonbenzodiazepine drug has also shown an increased rate of cancers in clinical trials. The review author concluded by saying: "The likelihood of cancer causation is sufficiently strong now that physicians and patients should be warned that hypnotics possibly place patients at higher risk for cancer".
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