Metformin is the first line option¹ unless there are contraindications (see BNF).

The aim of treatment is to reduce HbA1c to agreed target level in order to reduce long term complications from T2DM (see the table on benefits of long-term HbA1c reduction in the section on benefits of improved glycaemic control).

Benefits

• Metformin is effective, safe, inexpensive and may reduce risk of cardiovascular events and death.³⁶
• Compared with sulfonylureas, metformin as first-line therapy has beneficial effects on HbA1C, weight and cardiovascular mortality and has reduced risk of hypoglycaemia.³⁷

Many of the recent cardiovascular outcome trials compared new therapies added to metformin and not as first line options.

Side effects include:

• Gastrointestinal symptoms such as diarrhoea. This can be minimised by gradual increase of the dose when titrating to the dose required. A trial of metformin modified-release preparations could be considered according to local formulary guidance. 
• Associated with vitamin B₁₂ deficiency. This suggests that periodic testing of vitamin B₁₂ levels should be considered in metformin-treated patients, especially in those with anaemia or peripheral neuropathy.³⁸

Prescribing notes

• Metformin may be safely used in people with estimated glomerular filtration rate (eGFR) greater than 30 ml/min/1.73m² (dose adjustments required if eGFR less than 45ml/min).
  • Note that the BNF recommends exceptions to the use of eGFR include toxic drugs, in elderly patients and in patients at extremes of muscle mass where calculation of CrCl is recommended.³⁹
• Individuals should be advised to withhold metformin in cases of nausea, vomiting or dehydration (see Sick Day guidance)
• If metformin has been withheld due to acute kidney injury/inter-current illness, it can safely be restarted if the renal impairment has resolved.