Metformin: clinical use

Prescribing considerations

• Assess renal function before initiation and regularly thereafter, with more frequent monitoring in older people and those at risk of deterioration.
• Temporarily withhold during significant dehydration or acute illness likely to impair renal function; review before restarting.
• Plan interruption around intravascular iodinated contrast and surgery involving general, spinal or epidural anaesthesia according to product information and reassessed renal function.
• Consider vitamin B12 testing for anaemia, neuropathy or other suggestive symptoms, and periodic monitoring where risk factors are present.
• Reassess hypoglycaemia risk when combined with insulin or insulin secretagogues.

Contraindications and cautions

• Hypersensitivity to metformin or product excipients.
• Acute metabolic acidosis, including diabetic ketoacidosis or lactic acidosis, and diabetic pre-coma.
• Severe renal impairment; verify the product because thresholds differ. Xynox and several selected products contraindicate use below GFR 30 mL/min, while selected Colonis 850 mg/5 mL and 1000 mg/5 mL products specify CrCl or eGFR below 45.
• Acute conditions likely to impair renal function, including dehydration, severe infection or shock.
• Acute or worsening disease causing tissue hypoxia, including decompensated heart failure, respiratory failure, recent myocardial infarction or shock.
• Hepatic insufficiency, acute alcohol intoxication or alcoholism.

Monitoring

• Renal function at baseline and periodically, more frequently where deterioration is likely.
• Glycaemic response using appropriate laboratory measures.
• Vitamin B12 when deficiency is suspected; consider periodic testing for at-risk patients.
• Hydration and renal status during intercurrent illness or when starting medicines that may impair kidney function.
• Cardiac and renal function in stable chronic heart failure.

Clinical pharmacology

Metformin is an antihyperglycaemic biguanide. It suppresses hepatic glucose production, improves peripheral insulin sensitivity and delays intestinal glucose absorption. It is eliminated unchanged through the kidneys, making renal function central to safe use.

Formulation and product differences

• Selected immediate-release tablets, oral solutions and powder include licensing for adults and children from 10 years.
• Selected Xynox prolonged-release tablets are licensed for adults, must be swallowed whole and have a defined high-risk diabetes-prevention indication.
• Oral liquids differ in concentration, excipients, measuring devices, shelf life after opening and feeding-tube suitability; they are not automatically interchangeable volume-for-volume.
• Powder sachets must be dissolved in water before use.
• Prolonged-release tablet shells may appear in faeces; this does not usually mean the medicine was not released.

Modified-release tablet

Route: Oral

Strengths: 500 mg, 750 mg, 850 mg, 1000 mg

Oral solution

Route: Oral

Strengths: 500 mg, 500 mg/5 mL, 850 mg/5 mL, 1000 mg/5 mL