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Biguanide Pregnancy: B — used in gestational diabetes in some guidelines

Metformin

Brand names: Glucophage, Metabet

Adult dose

Dose: 500mg–1g twice daily
Route: Oral
Frequency: Twice daily with meals
Start 500mg OD/BD with food. Titrate monthly. Max 2g/day in divided doses. Modified-release: 500mg–2g once daily with evening meal.

Clinical pearls

  • First-line oral agent in T2DM regardless of HbA1c (unless contraindicated)
  • Weight-neutral or slight weight loss — advantage over sulfonylureas
  • Check B12 annually after 4+ years of use — causes B12 malabsorption
  • Modified-release formulation better tolerated GI-wise
  • Hold perioperatively and during acute illness (dehydration risk)

Contraindications

  • eGFR <30 mL/min/1.73m²
  • Acute kidney injury
  • Hepatic failure
  • Contrast media (hold 48h before/after)
  • Acute illness with dehydration

Side effects

  • GI upset (nausea, diarrhoea — dose-related)
  • Vitamin B12 deficiency (long-term)
  • Lactic acidosis (rare, usually with contraindications)
  • Metallic taste

Interactions

  • Iodinated contrast — hold for 48h post-contrast in eGFR <60
  • Alcohol — increased lactic acidosis risk

Monitoring

  • HbA1c (every 3 months until stable, then 6-monthly)
  • eGFR annually
  • Vitamin B12 annually

Reference: NICE NG28; ADA Standards of Care 2025. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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