Opioids may promote tumours

In one study, based at the University of Chicago, overexpression of opioid receptors in a lung cancer cell line increased in vitro and in vivo measures of tumour growth and metastasis. 

In another study, based at the University of North Carolina at Chapel Hill, naturally occurring genetic differences in opioid receptor biology correlated to breast cancer survival in a cohort of more than 2000 women. 

Breast cancer mortality was significantly reduced (at least halved) in patients with a genetic variant in the opioid receptor that reduces opioid response. 

Several studies have shown a reduced incidence of cancer recurrence after reduced doses of opioids post-surgery for breast, prostate and colon cancer and melanoma, although others have not detected any significant differences, the authors of the lung cancer study noted. 

They said their results implicated opioids in these epidemiological findings.