6.1.2.1 Biguanides

ChoiceDrugDosage
Formulary choiceMetformin tablets 500mg (immediate release)

Dose:
Initially 500mg with breakfast for at least 1 week, then 500mg with breakfast and evening meal for at least 1 week, then 500mg with breakfast, lunch and evening meal; usual max. 2g daily in divided doses
Caution in renal impairment
(see notes below)

Prescribing Notes

  • Metformin is the first choice oral antidiabetic drug. It is the only oral antidiabetic drug which has a proven survival advantage. It does not need to be limited to overweight patients.
  • Metformin may cause gastro-intestinal adverse effects; it should be started at low dose and taken with or after meals. A slow increase in dose may improve gastrointestinal tolerability.
  • Hypoglycaemia is not a problem with metformin monotherapy.
  • Metformin prolonged release tablets are more expensive than immediate release tablets, but less expensive than other newer oral agents. Metformin prolonged release tablets should be reserved for patients:
    • unable to tolerate immediate release metformin, or
    • with demonstrable compliance problems (once daily dosing)
    • if metformin MR is necessary, Sukkarto® SR is the recommended cost-effective choice
  • Metformin is a useful drug and can be safely used in mild/moderate stable CKD. It is associated with lactic acidosis, however this is rare and the risk may be overstated. It is reasonable for GPs to use metformin in people with Stage 3 CKD (eGFR >30mL/min). However, dose reduction and specialist involvement should be considered as renal function declines towards this level.
  • Continuing metformin during periods of dehydration or acute illness (such as diarrhoea and vomiting) can increase the risk of lactic acidosis. This is compounded if the patient is also taking diuretics, ACE inhibitors, ARBs, NSAIDs and/or SGLT2 inhibitors in combination with metformin. Unlike acute illnesses in type 1 diabetes (where insulin treatment must be continued), stopping the drugs for a day or two will not cause any immediate problem for the patient and will protect renal function until the patient improves.