Metformin for Type 2 Diabetes: What Australians Need to Know

Metformin is the most prescribed medication for type 2 diabetes worldwide and has been a cornerstone of treatment for over 60 years. In Australia, the Therapeutic Guidelines (eTG) recommend it as the first-line pharmacological therapy for type 2 diabetes, alongside lifestyle modifications. Over 2 million Australians have type 2 diabetes, and metformin is typically the first medication prescribed after diagnosis.

How Metformin Works

Metformin is a biguanide that works primarily by reducing glucose production in the liver and improving insulin sensitivity in muscle tissue. Unlike some diabetes medications, metformin does not stimulate insulin secretion, which means it rarely causes hypoglycaemia (dangerously low blood sugar) when used alone.

• Reduces hepatic (liver) glucose production -- the primary effect
• Improves insulin sensitivity in peripheral tissues (muscle, fat)
• Modestly reduces intestinal absorption of glucose
• Does not cause weight gain -- may cause modest weight loss (1-2kg on average)
• Does not stimulate insulin release -- very low risk of hypoglycaemia as monotherapy

Dosing and Formulations

Metformin is available in immediate-release (IR) and modified-release (MR/XR) tablets. The modified-release formulation was developed to reduce gastrointestinal side effects, which are metformin's main tolerability issue.

• Immediate-release (IR): 500mg, 850mg, 1000mg tablets -- taken 2-3 times daily with meals
• Modified-release (MR/XR): 500mg, 1000mg tablets -- taken once daily (usually with evening meal)
• Starting dose: typically 500mg once or twice daily, increasing gradually over 2-4 weeks
• Target dose: usually 1500-2000mg daily (the dose at which maximum glucose-lowering effect is seen)
• Maximum dose: 3000mg daily (IR) or 2000mg daily (MR)

Side Effects

Common (10-30% of patients)

• Nausea, particularly when starting or increasing the dose
• Diarrhoea or loose stools
• Abdominal cramping or bloating
• Metallic taste
• Reduced appetite

GI side effects typically improve after 2-4 weeks as the body adjusts. Taking metformin with food helps significantly. If IR tablets cause persistent GI issues, switching to the modified-release (MR) formulation often resolves them.

Rare but serious

• Vitamin B12 deficiency: occurs in 5-10% of long-term users. Annual B12 monitoring is recommended, particularly after 3+ years of use
• Lactic acidosis: extremely rare (estimated 3-10 cases per 100,000 patient-years) but potentially fatal. Risk is elevated by severe kidney impairment, liver disease, alcohol excess, acute illness, or dehydration

Kidney Function and Metformin

Metformin is cleared by the kidneys. If kidney function declines (as measured by eGFR), the dose may need to be reduced or the medication stopped to avoid accumulation and lactic acidosis risk.

• eGFR > 60: full dose (up to 2000-3000mg daily)
• eGFR 30-60: reduce dose; maximum 1000mg daily; monitor kidney function 3-6 monthly
• eGFR < 30: contraindicated -- metformin should be stopped

PBS Status and Cost

Metformin is listed on the PBS for type 2 diabetes. This makes it one of the most affordable diabetes medications in Australia.

• General patients: PBS co-payment (up to $31.60 for 2024)
• Concession card holders: $7.70 per script
• Generic metformin: widely available and bioequivalent to branded versions
• Cost without PBS (private): approximately $10-$25 for a month's supply

Lifestyle Alongside Metformin

Metformin is not a replacement for lifestyle management -- it is an addition. The Australian Therapeutic Guidelines emphasise that diet, exercise, and weight management remain the foundation of type 2 diabetes treatment. Metformin is most effective when combined with these lifestyle modifications.

• Regular physical activity: 150 minutes per week of moderate-intensity exercise (eTG recommendation)
• Dietary management: reduced refined carbohydrates, increased fibre, portion control
• Weight loss: even 5-10% body weight reduction significantly improves glycaemic control
• Alcohol moderation: alcohol increases lactic acidosis risk and can cause hypoglycaemia
• Regular monitoring: HbA1c every 3-6 months, annual kidney function, annual B12 after 3 years

When Metformin Is Not Enough

If metformin alone does not achieve glycaemic targets (typically HbA1c below 53 mmol/mol or 7%), additional medications may be added. Second-line options in Australia include SGLT2 inhibitors (e.g., empagliflozin, dapagliflozin), GLP-1 receptor agonists (e.g., semaglutide, dulaglutide), DPP-4 inhibitors, and sulfonylureas. The choice depends on individual factors including cardiovascular risk, weight, kidney function, and cost. Your GP or endocrinologist will guide this decision.

Getting Metformin Prescribed

Metformin requires a prescription from an AHPRA-registered doctor. For initial diagnosis and treatment commencement, an in-person GP consultation is recommended to include blood tests (HbA1c, fasting glucose, kidney function, lipids). For ongoing repeat prescriptions of stable, well-managed type 2 diabetes, telehealth is a convenient option.