Metformin is more effective and has fewer side-effects than newer and more expensive medications, according to a US review by Johns Hopkins University researchers.
The analysis, which adds weight to previous findings, included 140 trials and 26 observational studies.
Most medications and two-drug combinations produced about the same decrease in HbA1c levels, but other outcomes differed considerably.
The analysis found that evidence for long-term clinical outcomes, such as mortality, cardiovascular disease or neuropathy, was weak or insufficient.
Metformin was more effective than DPP-4 inhibitors and less likely to cause weight gain than thiazolidinediones or sulfonylureas.
Sulfonylureas had four times the risk of mild or moderate hypoglycaemia than metformin alone.
Thiazolidinediones increased the risk for congestive heart failure compared with sulfonylureas and increased risk for