free full access: Diabetes and Cancer: A Consensus Report -- A Cancer Journal for Clinicians

"Because specific antihyperglycemic medications are associated with cancer risk factors, confounding by unmeasured or incompletely measured risk factors may explain at least in part the previously reported associations between medications and cancer. To our knowledge, few studies to date have examined the risk associated with the dose, duration, or recency of medication use, which might inform the biologic plausibility of observed associations. Many agents that affect carcinogenesis have long latencies or require a minimum exposure level, and the risk associated with some agents may return to baseline after the exposure has been terminated for a period of time. Some diabetes medications have only recently come on the market (eg, TZDs, insulin analogs, and incretin-based therapies), and therefore, studies of these agents will only assess the cancer risk associated with relatively short-term use.

It is unlikely that the effect of diabetes therapies on cancer risk and progression, particularly at specific cancer sites, will be fully addressed with randomized controlled clinical trials, due to both cost and follow-up time limitations.
Such trials would also be confounded by the natural crossover and treatment escalation required to appropriately treat progressive hyperglycemia. Given these limitations, multiple well-conducted and appropriately designed prospective observational studies are needed. Results of in vitro and preclinical studies should inform design considerations for observational studies, but by themselves cannot be considered conclusive."