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Biguanide Pregnancy: A large amount of data on pregnant women (more than 1000 exposed outcomes) indicates no increased risk of congenital abnormalities or feto/neonatal toxicity after exposure in the periconceptional phase and/or during pregnancy; if clinically needed, metformin can be considered during pregnancy as an addition or alternative to insulin. Metformin crosses the placenta at levels that can be as high as maternal concentrations. Breast-feeding is not recommended during metformin treatment as only limited data are available.

Metformin

Brand names: Glucophage, Metabet

Used in: Diabetes & DKA

Metformin is a biguanide and the usual first-line oral medicine for type 2 diabetes. Used alone it lowers blood glucose without causing hypoglycaemia or weight gain, and it has a long safety record.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 500 mg or 850 mg metformin hydrochloride
Route: Oral
Frequency: 2 or 3 times daily, given during or after meals
Max: 3 g metformin hydrochloride daily, taken as 3 divided doses
UK SPC section 4.2, adults with normal renal function (GFR at least 90 mL/min), monotherapy or combination with other oral antidiabetic agents. After 10 to 15 days the dose should be adjusted on the basis of blood glucose measurements; a slow increase of dose may improve gastrointestinal tolerability. If transferring from another oral antidiabetic agent, discontinue the other agent and initiate metformin at the dose above. Combination with insulin: metformin is given at the usual starting dose of 500 mg or 850 mg 2 or 3 times daily while the insulin dose is adjusted on the basis of blood glucose measurements. Elderly: adjust the dose based on renal function, with regular assessment of renal function. Paediatric population: metformin can be used in children from 10 years of age and adolescents - usual starting dose 500 mg or 850 mg once daily during or after meals, adjusted after 10 to 15 days on the basis of blood glucose, to a maximum recommended dose of 2 g daily taken as 2 or 3 divided doses; verify paediatric dosing against a children's formulary. Discontinue prior to or at the time of intravascular administration of iodinated contrast agents (risk of contrast-induced nephropathy leading to metformin accumulation and lactic acidosis).

Dose adjustments

Renal

Assess GFR before initiation and at least annually thereafter (every 3 to 6 months in patients at increased risk of progression and in the elderly). Total maximum daily dose, divided into 2 to 3 daily doses: GFR 60-89 mL/min - 3000 mg (dose reduction may be considered in relation to declining renal function); GFR 45-59 mL/min - 2000 mg (review factors that may increase the risk of lactic acidosis before initiation; the starting dose is at most half of the maximum dose); GFR 30-44 mL/min - 1000 mg; GFR below 30 mL/min - metformin is contraindicated. Temporarily discontinue in the presence of conditions that alter renal function.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

  • Hypersensitivity to the active substance or to any of the excipients
  • Any type of acute metabolic acidosis (such as lactic acidosis, diabetic ketoacidosis)
  • Diabetic pre-coma
  • Severe renal failure (GFR below 30 mL/min)
  • Acute conditions with the potential to alter renal function such as dehydration, severe infection or shock
  • Disease which may cause tissue hypoxia (especially acute disease or worsening of chronic disease) such as decompensated heart failure, respiratory failure or recent myocardial infarction
  • Hepatic insufficiency, acute alcohol intoxication, alcoholism

Side effects

  • Very common: gastrointestinal disorders such as nausea, vomiting, diarrhoea, abdominal pain and loss of appetite, most frequent at initiation and usually resolving spontaneously
  • Common: vitamin B12 decrease/deficiency
  • Common: taste disturbance
  • Very rare: lactic acidosis
  • Very rare: liver function test abnormalities or hepatitis, resolving on discontinuation
  • Very rare: skin reactions such as erythema, pruritus, urticaria

Interactions

  • Medicinal products that can acutely impair renal function (antihypertensives, diuretics, NSAIDs): initiate with caution in metformin-treated patients (UK SPC section 4.4)
  • Iodinated contrast agents: intravascular administration may cause contrast-induced nephropathy, metformin accumulation and increased risk of lactic acidosis - metformin should be discontinued prior to or at the time of the imaging procedure (UK SPC section 4.4)
  • Alcohol: potentiates the effect of metformin on lactate metabolism; excessive alcohol intake is a risk factor for lactic acidosis
  • Carbonic anhydrase inhibitors (topiramate, zonisamide, acetazolamide, dichlorphenamide): decrease serum bicarbonate and may increase the risk of lactic acidosis - consider more frequent monitoring (US labelling)
  • Drugs that reduce metformin clearance (OCT2/MATE inhibitors such as ranolazine, vandetanib, dolutegravir, cimetidine): increased systemic metformin exposure and risk of lactic acidosis (US labelling)
  • Insulin or insulin secretagogues: increased risk of hypoglycaemia - a lower dose of insulin or secretagogue may be required (US labelling)

Clinical monograph

How it works

Metformin reduces hepatic gluconeogenesis and increases peripheral insulin sensitivity and glucose uptake (in part via AMP-activated protein kinase). It does not stimulate insulin secretion.

Prescribing in practice

  • Introduce gradually with food to limit gastrointestinal effects; a modified-release form can improve tolerability.
  • Dosing is limited by renal function — review eGFR before and during treatment, and reduce or stop at low eGFR (see the Renal Drug Dosing tool).
  • Withhold during acute illness with dehydration or hypoxia, and around iodinated contrast or major surgery, because of the risk of lactic acidosis.
  • Long-term use can lower vitamin B12 absorption; check B12 if features of deficiency develop.

Monitoring

Monitor HbA1c for glycaemic response, and renal function at least annually (more often if impaired or on interacting drugs).

Counselling the patient

  • Take with or after food to reduce stomach upset, which usually settles.
  • On its own it does not usually cause low blood sugar.
  • Stop and seek advice if you become acutely unwell or dehydrated, or before procedures using contrast dye.

Evidence & guidelines

First-line for type 2 diabetes in NICE NG28 unless contraindicated or not tolerated; cardiovascular benefit in overweight patients was shown in UKPDS.

Reference: NICE NG28; ADA Standards of Care 2025; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

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