Biguanide
Metformin hydrochloride
Activates AMP-kinase, reducing hepatic gluconeogenesis and increasing peripheral insulin sensitivity and glucose uptake; does not stimulate insulin secretion, so it does not cause hypoglycaemia alone.
Brands
Glucophage · Glycomet · Fortamet
Adult dosing
Maximum 2 g/day (immediate or modified release)| Indication | Dose | Route | Frequency |
|---|---|---|---|
| Type 2 diabetes — immediate release, initiation | 500 mgIncrease by 500 mg each week to limit gastrointestinal effects | Oral | Once daily with the evening meal |
| Type 2 diabetes — usual maintenance | 500 mg-1 gMax 2 g/day in 2-3 divided doses | Oral | Twice daily with food |
| Type 2 diabetes — modified release | 500 mg-2 gMax 2 g once dailySwitch from immediate release at the same total daily dose | Oral | Once daily with the evening meal |
| Polycystic ovary syndrome with insulin resistance | 500 mgMax 1.5-2 g/day | Oral | Two to three times daily |
Weight-based: Not weight based in adults or children — titrate to tolerance and HbA1c.
Older adults: No age-based reduction, but dose by measured eGFR rather than creatinine alone; sarcopenia hides true renal impairment.
Paediatric dosing
Always confirm the weight and the local paediatric formulary| Indication | Dose | Route | Frequency |
|---|---|---|---|
| Type 2 diabetes, 10 years and older | 500 mgMax 2 g/day in divided dosesNot licensed below 10 years; modified release is generally avoided in children | Oral | Once daily, increased weekly by 500 mg |
| Adolescent with obesity-related insulin resistance (specialist use) | 500 mg-1 gMax 2 g/day | Oral | Twice daily with food |
Renal adjustment
Move the slider to the patient's eGFRFull dose, up to 2 g/day
Recheck eGFR at least annually
- eGFR ≥ 60Full dose, up to 2 g/day
- eGFR 45-59Full dose, up to 2 g/day
- eGFR 30-44Maximum 1 g/day in 2 divided doses; halve an existing dose
- eGFR < 30Stop — contraindicated
Hepatic adjustment
- Mild impairmentUsual dose with review of alcohol intake
- Moderate to severe impairment or active alcohol misuseAvoid — impaired lactate clearance
Practical notes
The things that go wrong at the bedside- Withhold on the day of iodinated contrast if eGFR is below 60, and restart 48 hours later once renal function is confirmed stable.
- Withhold during any acute illness with dehydration, hypoxia, sepsis or fasting for surgery.
- Give with or just after food; a modified-release switch resolves most gastrointestinal intolerance.
- Check vitamin B12 every 2-3 years, or sooner with neuropathy or anaemia.
DrZep structured dosing table (demo dataset, formulary-aligned) · DrZep. Confirm every dose against your local formulary and the patient's weight, renal function and comorbidities.
Indications
- First-line pharmacotherapy for type 2 diabetes
- Prediabetes in selected high-risk individuals
- Polycystic ovary syndrome with insulin resistance
- Antipsychotic-associated weight gain (off-label)
Dosing summary
- Start
- 500 mg once or twice daily with food
- Titration
- Increase by 500 mg weekly to limit GI effects
- Usual maintenance
- 1 g twice daily
- Maximum
- 2 g/day (extended release up to 2 g once daily)
Organ adjustment
Renal
- eGFR ≥ 60
- No adjustment
- eGFR 45-59
- Continue; monitor renal function every 3-6 months
- eGFR 30-44
- Maximum 1 g/day; do not initiate
- eGFR < 30
- Contraindicated
- Acute illness / dehydration / contrast
- Withhold temporarily
Hepatic
- Significant hepatic impairment
- Avoid — increased lactic acidosis risk
Monitoring
- 1HbA1c every 3-6 months
- 2eGFR at least annually, more often if 30-59
- 3Vitamin B12 every 2-3 years or with neuropathy or anaemia
Contraindications & cautions
Do not use / use with care
- eGFR < 30 mL/min/1.73 m²
- Acute metabolic acidosis including DKA
- Acute decompensated heart failure or shock
- Severe hepatic impairment or active alcohol misuse
Pregnancy & lactation
- Pregnancy
- Used in gestational and pre-existing type 2 diabetes in many settings; insulin remains the reference standard — follow local obstetric guidance.
- Lactation
- Compatible; minimal milk transfer.
Interactions
- Iodinated contrast mediaRisk of contrast-associated AKI and lactic acidosis — withhold around the procedure per eGFRmajor
- Alcohol (acute excess)Increased lactic acidosis riskmajor
- Cimetidine, dolutegravir, ranolazineReduced renal clearance, higher metformin levelsmoderate
- Loop diuretics, ACE inhibitors, NSAIDsRenal hypoperfusion increasing accumulation riskmoderate
Adverse effects
Common
- Diarrhoea
- Nausea and abdominal discomfort
- Metallic taste
- Reduced appetite
Serious
- Lactic acidosis (rare but high mortality)
- Vitamin B12 deficiency with long-term use
Toxicity & overdose
Recognise and treat
- Metformin-associated lactic acidosis: high anion gap acidosis with lactate > 5, often precipitated by AKI or sepsis
- Treat with supportive care and haemodialysis, which clears both metformin and lactate
Alternatives
- SGLT2 inhibitor (e.g. empagliflozin)
- GLP-1 receptor agonist
- DPP-4 inhibitor
- Sulfonylurea where cost dominates
Linked diseases & guidelines
Demo monograph for educational use. Always confirm dose, route and interactions against your national formulary and the product label before prescribing.
Demo content. Educational decision support only. Verify every dose, citation and recommendation against your national formulary and the primary source before clinical use.