title: Metformin and cancer in type 2 diabetes: a systematic review and comprehensive bias evaluation.
abstract: Existing observational studies provide conflicting evidence for the causal effect of metformin use on cancer risk in patients with type-2 diabetes, and there are concerns about bias affecting a number of studies.
  
  
  
  MEDLINE was used to identify observational studies investigating the association between metformin and overall or site-specific cancer in people with type-2 diabetes. A systematic data extraction and bias assessment was conducted, in which risk of eight bias domains (outcome, exposure, control selection, baseline confounding, time-dependent confounding, immortal time, missing data, censoring methods) were assessed against pre-defined criteria, and rated as unlikely, low, medium or high.
  
  
  
  Of 46 studies identified, 21 assessed the effect of metformin on all cancer. Reported relative risks ranged from 0.23 to 1.22, with 12/21 reporting a statistically significant protective effect and none a harmful effect. The range of estimates was similar for site-specific cancers; 3/46 studies were rated as low or unlikely risk of bias in all domains. Two of these had results consistent with no effect of metformin; one observed a moderate protective effect overall, but presented further analyses that the authors concluded were inconsistent with causality. However, 28/46 studies were at risk from bias through exposure definition, 22 through insufficient baseline adjustment and 35 from possible time-dependent confounding.
  
  
  
  Observational studies on metformin and cancer varied in design, and the majority were at risk of a range of biases. The studies least likely to be affected by bias did not support a causal effect of metformin on cancer risk.
authors.affiliation: Department of Non-communicable Disease Epidemiology, London School of Hygiene and Tropical Medicine, London, UK.|MRC Clinical Trials Unit at UCL, London, UK and.
authors.forename: Ruth E
authors.initials: RE
authors.lastname: Farmer
authors.affiliation: MRC Clinical Trials Unit at UCL, London, UK and.
authors.forename: Deborah
authors.initials: D
authors.lastname: Ford
authors.affiliation: Department of Non-communicable Disease Epidemiology, London School of Hygiene and Tropical Medicine, London, UK.
authors.forename: Harriet J
authors.initials: HJ
authors.lastname: Forbes
authors.affiliation: Institute of Cardiovascular Science, University College London, London, UK.
authors.forename: Nishi
authors.initials: N
authors.lastname: Chaturvedi
authors.affiliation: MRC Clinical Trials Unit at UCL, London, UK and.
authors.forename: Richard
authors.initials: R
authors.lastname: Kaplan
authors.affiliation: Department of Non-communicable Disease Epidemiology, London School of Hygiene and Tropical Medicine, London, UK.
authors.forename: Liam
authors.initials: L
authors.lastname: Smeeth
authors.affiliation: Department of Non-communicable Disease Epidemiology, London School of Hygiene and Tropical Medicine, London, UK.
authors.forename: Krishnan
authors.initials: K
authors.lastname: Bhaskaran
mesh_terms: D015982:Bias; D003924:Diabetes Mellitus, Type 2 / Q000150:complications* / Q000188:drug therapy*; D006801:Humans; D007004:Hypoglycemic Agents / Q000009:adverse effects* / Q000627:therapeutic use; D008687:Metformin / Q000009:adverse effects* / Q000627:therapeutic use; D009369:Neoplasms / Q000453:epidemiology*; D064887:Observational Studies as Topic
chemical_list: D007004:Hypoglycemic Agents; D008687:Metformin
keywords: Pharmacoepidemiology; bias; cancer; causality; confounding; diabetes