The Diabetes Prevention Program trial compared metformin, lifestyle modification and placebo (the latter, as with metformin, including basic lifestyle advice). Both metformin and lifestyle alternatives reduced diabetes versus placebo. A published cost-effectiveness analysis (CEA) of the DPP as if it were implemented in Singapore concluded that both metformin and lifestyle were cost-effective. The original work miscalculated the key metric in economic evaluations – the Incremental Cost-Effectiveness Ratio (ICER) - by comparing metformin and lifestyle each to a common alternative (here, placebo), a violation of long-established economic methods. We revisited that analysis, using the identical data, but with appropriate methods, to calculate appropriate ICERs. We showed that with correctly calculated ICERs, metformin was not even technically efficient and thereby incapable of being the more-restrictive economically efficient (cost-effective) treatment. These data show that only lifestyle was cost-effective. We also expanded the analysis, using three additional methods (Incremental Cost-Effectiveness Plane, Net Monetary Benefits and Net Loss Curves). These methods confirm the results from the correct ICERs. Had any of these been used in the original CEA, errors may have been identified by the authors. Importantly, our Net Loss calculations also show that there may be significant health and cost consequences to patients in a system that may have implemented policy based on the error that incorrectly implied metformin’s cost-effectiveness. Policymakers may easily be misled by peer-reviewed published economic evaluations that fail to follow appropriate economic methods and lead them to implement policies that are harmful and costly relative to using cost-effective treatments.