I'll bet when you read that title, you thought, "Oh no! Another drug that has been linked with cancer."
But in this case, the opposite is true. There is some evidence that the drug metformin actually decreases your risks of breast cancer. The full article in PloS ONE cites references discussing metformin and other cancers. And here is a 2010 meta-analysis of metformin and various types of cancer. Interestingly, this study showed no significant association between breast cancer and metformin, although there was an insignificant trend in that direction.
This is not actually big news. It was shown several years ago that there was a statistical association between cancer and long-term metformin usage; the metformin usage was associated with less cancer. But showing a statistical association doesn't always mean a cause-and-effect relationship, as I discuss here.
What is new about this study is an attempt to show how the metformin has an effect on breast cancer. Estrogen and other known chemicals that promote breast cancer caused breast tumors to increase in size and numbers in vitro. When metformin was added, the tumors grew less.
These days, there's so much bad news about drugs, it's nice to see some good news for a change. Generic metformin is cheap, seems to be effective in type 2 (and even helps in some type 1 patients), and now seems to have a good effect on cancer risks.
One caveat. Endocrinologist Dr. Richard K. Bernstein, author of The Diabetes Solution," says that generic metformin doesn't always work as well as the original drug, Glucophage. The evidence is annecdotal. He had patients on Glucophage, and when the generics came out and they switched to those, some patients saw their blood glucose levels increase. When they went back to Glucophage, the levels went down again.
This could have been simply one generic version, or a problem with the older generics, or something special about these patients. Bernstein hasn't researched it.
Generics are required to contain the same active drug as the brand name drugs. But the vehicle for the drug, fillers, and so forth, can be different. In some cases, a generic drug might not dissolve as well, or an extended-release product might not dissolve as evenly.
I once opened capsules from Prilosec and the generic version. The Prilosec had tiny spheres, all the same size. The generic contained little globs of various sizes and shapes. Obviously the machine that made the stuff wasn't as well machined. Would this variation in size affect the release of the drug? I really don't know.
But this does mean that if you're prescribed generic metformin and it doesn't work very well for you, one possibility is that you're not really type 2. But another possibility is that the generic version you got doesn't work very well for you. In that case, you might want to do a trial of Glucophage.
The problem is the availability and price. Glucophage isn't on the formulary of my Plan D. If I buy it at the drug store, it's about $80 for a month's supply. I pay $20 for a 3-month supply of the generic, or about $7 a month. That's a big difference. Is it worth it? If the Glucophage worked better, then yes. But with so many other variables in my regimen, it would be difficult to know (unless the effect were enormous) without a long-term trial keeping everything else constant, which is difficult to do with my lifestyle, and statistical analysis.
For now, I'll console myself with the thought that at least metformin seems to be the best choice of a drug for type 2.