But the latest research has convinced Pollak and some others that cancer treatment should be taken off the list.
More studies, but no proof
One of the first indications that metformin is linked to anticancer effects appeared in a brief note in the British Medical Journal in 2005. Researchers analyzed the medical records of almost 12,000 people from the Tayside region of Scotland who had recently been diagnosed with diabetes between 1993 and 2001. , more than 900 developed cancer. Interestingly, those who had used metformin at any time during the study period were 23 percent less likely to receive a cancer diagnosis later.
This finding led to further research into people with diabetes who took metformin and into the risk of breast cancer, liver cancer, ovarian and endometrial cancer, and other types of cancer. The authors of a 2013 analysis that included more than 1 million patients in 41 observational studies like the original concluded that metformin “may be associated with a significant reduction in cancer risk.” But such associations are not proof.
Researchers went on to investigate the link in studies using cells in dishes and in laboratory animals, finding that metformin slowed the growth of blood, breast, endometrial, lung, liver, stomach and thyroid cancer cells. It also seemed to make cancer cells extra sensitive to chemotherapy drugs. In one mouse study, scientists transplanted human breast, prostate, or lung cancer cells into the animals and treated them with standard chemotherapy drugs, metformin, or a combination of both. The combination worked best, preventing tumor growth and prolonging the relapse.
These findings made sense to researchers. Metformin treats metabolic problems in diabetes, and cancer has also been linked to metabolic problems such as obesity. Even before the 2005 British Medical Journal study, Goodwin had noted that breast cancer patients with high insulin levels fared worse than those with normal insulin levels.
That logic, plus the promising data, led scientists to conduct a number of randomized controlled trials—the gold standard experiment in medicine. Researchers would enroll people with cancer and divide them into two groups. One group would receive standard cancer therapy plus metformin; the other group received standard therapy plus a placebo, a pill without drugs.
