Metformin and Cancer (Continued)
Keen Observations: Retrospective Research
First approved in the United States in 1994, metformin has since become a widely used treatment for type 2 diabetes, largely due to its efficacy, safety, cost, and ease of use. In addition to diet and exercise, it is recommended as first-line therapy by many medical organizations, including ADA, EASD, and IDF. The drug is estimated to be the most widely prescribed diabetes medication worldwide, with over 40 million prescriptions filled in 2008 in the US alone. For this reason, there are towering stacks of historical medical records for type 2 diabetes patients treated with metformin, ripe for analysis and research.
The four articles published in Diabetologia were retrospective in nature, meaning the studies compiled and analyzed data from already established patient records. The most suggestive study of the four, conducted by Dr. Craig Currie and colleagues (Cardiff University), reviewed a total of 63,000 people with diabetes from the THIN database, a collection of over five million patient records from medical practices across the UK. Analyses suggested patients treated with metformin alone when compared with sulfonylureas and insulin carried the lowest risk of cancer; furthermore, patients taking metformin in addition to insulin therapy showed a 46% decline in cancer risk compared to those on insulin therapy alone.
Other retrospective studies have shown similar findings. A recent review of 400 people with diabetes from the University of Texas MD Anderson Cancer Center suggested a 62% lower risk of pancreatic cancer in patients who took metformin compared to those who did not; an older study conducted at the University of Dundee in Scotland showed a 25-37% reduced risk for all cancers in people with diabetes taking metformin. Notably, research has also shown that people with diabetes already diagnosed with cancer may respond better to chemotherapy when treated simultaneously with metformin. Taking metformin may also reduce the dose of noxious chemotherapeutic agents in patients undergoing cancer treatment, easing some of the distressing side effects brought by chemotherapy.
However, it is important to keep in mind the weaknesses of retrospective and observational studies. Unlike randomized clinical trials, the "gold standard" of medical research, retrospective studies do not randomize subjects to roughly equal groups before treatment begins, leaving room for external factors (such as social background, previous illnesses, etc.) to confound results. Records databases can be biased by differences in the way medical practices report information about patients as well. Thus, it is unclear if metformin is directly responsible for the decreased cancer risk. Researchers are also still unsure how the drug might biologically cause the observed effects, though hypotheses have been proposed.
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