Metformin is a drug that shows up in discussion here every so often. It is thought to be a calorie restriction mimetic, recapitulating some of the metabolic changes caused by the practice of calorie restriction. Its effects on life span in laboratory animals are up for debate and further accumulation of evidence - the results are on balance more promising than the generally dismal situation for resveratrol, but far less evidently beneficial than rapamycin. Like rapamycin, metformin isn't something you'd want to take as though it were candy, even if the regulators stood back to make that possible, as the side effects are not pleasant and potentially serious.
I should note as an aside that while ongoing research into the effects of old-school drugs of this nature is certainly interesting, it doesn't really present a path to significantly enhanced health and longevity. It is a pity that such research continues to receive the lion's share of funding, given that the best case outcome is an increase in our knowledge of human metabolism, not meaningful longevity therapies. Even if the completely beneficial mechanism of action is split out from the drug's actions - as seems to be underway for rapamycin - the end results will still only be a very modest slowing of aging. You could do better by exercising, or practicing calorie restriction.
For the billions in funding poured into these drug investigation programs, there should be a better grail at the end of the road - such as that offered by the SENS vision of rejuvenation biotechnology. Targeted repair of the biological damage of aging is a far, far better strategy than gently slowing the pace of damage accumulation through old-style drug discovery programs. This is a biotechnology revolution: time to start acting like it.
Anyway, aside done, let me point you to a recent open access review on metformin: the interesting work that won't really be in any way relevant to the future of your longevity, but which I'll wager has raised more funding as an object of study than the entire present extant SENS program and directly related scientific studies:
Metformin, an oral anti-diabetic drug, is being considered increasingly for treatment and prevention of cancer, obesity as well as for the extension of healthy lifespan. Gradually accumulating discrepancies about its effect on cancer and obesity can be explained by the shortage of randomized clinical trials, differences between control groups (reference points), gender- and age-associated effects and pharmacogenetic factors. Studies of the potential antiaging effects of antidiabetic biguanides, such as metformin, are still experimental for obvious reasons and their results are currently ambiguous.
The wave of interest, with periodical decays and increasing surges, was associated with the attempts to use antidiabetic biguanides [such as metformin] to control body weight and tumor growth. Another facet of the situation is that almost 45 years ago these drugs were suggested to promote longevity. Over the last years, the expanding bodies of relevant evidence, which mainly related to metformin, started to merge and occupy increasing place in current literature. The objective of the present essay is to attract more attention to accumulating inconsistencies. The first two sections of the essay, which are related to obesity and cancer, are based mostly on clinical data. The third section, which is related to aging or, rather, antiaging, is based predominately on experimental evidence obtained in rodents. Clearly, obesity and cancer have numerous interrelationships with aging, [however], we will separate these aspects for the sake of clarity in discussing the relevant effects of metformin.
See what you think; it makes for an interesting read - and includes a table of results from a number of life span studies that are, indeed, all over the map. It somewhat reinforces the point that unambiguous success in extending healthy life is not going to arrive from this quarter. Think SENS, not drug discovery - what will come from the drug discovery clade is a slow, grinding, and expensive cataloging of the fine details of genetics, metabolism, and aging in mammals.