An estimated 16·5 million people worldwide illicitly use opiates, of whom 4 million use raw opium. We did a systematic review to investigate the association between opium use and cancer incidence and mortality. Opium use was associated with an increased risk of cancers of the oesophagus, stomach, larynx, lung, and urinary bladder. Although the present evidence suggests that these associations are possibly causal, further epidemiological studies (particularly prospective studies that collect detailed data about lifetime opium use and control for a broad range of potential confounders) are needed.