Comparison of metformin and insulin versus insulin alone for type 2 diabetes: systematic review of randomised clinical trials with meta-analyses and trial sequential analyses.

OBJECTIVES: To compare the benefits and harms of metformin and insulin versus insulin alone as reported in randomised clinical trials of patients with type 2 diabetes.
DESIGN: Systematic review of randomised clinical trials with meta-analyses and trial sequential analyses. DATA SOURCES: The Cochrane Library, Medline, Embase, Science Citation Index Expanded, Latin American Caribbean Health Sciences Literature, and Cumulative Index to Nursing and Allied Health Literature until March 2011. We also searched abstracts presented at the American Diabetes Association and European Association for the Study of Diabetes Congresses, contacted relevant trial authors and pharmaceutical companies, hand searched reference lists of included trials, and searched the US Food and Drug Administration website. REVIEW
METHODS: Two authors independently screened titles and abstracts for randomised clinical trials comparing metformin and insulin versus insulin alone (with or without placebo) in patients with type 2 diabetes, older than 18 years, and with an intervention period of at least 12 weeks. We included trials irrespective of language, publication status, predefined outcomes, antidiabetic interventions used before randomisation, and reported outcomes.
RESULTS: We included 26 randomised trials with 2286 participants, of which 23 trials with 2117 participants could provide data. All trials had high risk of bias. Data were sparse for outcomes relevant to patients. Metformin and insulin versus insulin alone did not significantly affect all cause mortality (relative risk 1.30, 95% confidence interval 0.57 to 2.99) or cardiovascular mortality (1.70, 0.35 to 8.30). Trial sequential analyses showed that more trials were needed before reliable conclusions could be drawn regarding these outcomes. In a fixed effect model, but not in a random effects model, severe hypoglycaemia was significantly more frequent with metformin and insulin than with insulin alone (2.83, 1.17 to 6.86). In a random effects model, metformin and insulin resulted in reduced HbA(1c), weight gain, and insulin dose, compared with insulin alone; trial sequential analyses showed sufficient evidence for a HbA(1c) reduction of 0.5%, lower weight gain of 1 kg, and lower insulin dose of 5 U/day.
CONCLUSIONS: There was no evidence or even a trend towards improved all cause mortality or cardiovascular mortality with metformin and insulin, compared with insulin alone in type 2 diabetes. Data were limited by the severe lack of data reported by trials for patient relevant outcomes and by poor bias control.