Metformin: how it works, side effects and tips for taking it

By the Medsmingle Team · Updated 13 November 2026

This is a prescription medicine. Only take it if it has been prescribed for you, and follow your doctor’s and pharmacist’s instructions.

Metformin is a commonly prescribed medicine for type 2 diabetes. Treatment choice depends on individual health needs; metformin has been used for decades and doesn’t usually cause low blood sugar or weight gain on its own.

How it works

Metformin lowers the amount of sugar your liver releases into the blood and helps your body respond better to insulin.

Common side effects

• Nausea, upset stomach, diarrhea or gas, mostly in the first few weeks
• A metallic taste
• Reduced appetite

Tips to reduce stomach side effects

• Take it with meals
• Your doctor will usually start with a low dose and increase it slowly
• Ask about the extended-release (XR) form, which is often easier on the stomach

Vitamin B12

Long-term use can lower vitamin B12 levels. Ask your doctor about a B12 test, especially if you have tingling, numbness or tiredness.

Lactic acidosis: rare but serious

Very rarely, metformin can cause a build-up of lactic acid, mainly in people with serious kidney, liver or heart problems or heavy alcohol use. Get urgent help for unusual muscle pain, trouble breathing, severe tiredness, stomach pain with vomiting, or feeling cold and dizzy.

Important to know

• Your doctor will check your kidney function regularly
• Metformin may need to be paused for some contrast scans, procedures or acute illnesses. Ask your clinician what to do if you develop severe vomiting, diarrhea or dehydration; do not make routine changes on your own.
• Avoid heavy drinking
• Tell your doctor about all other medicines

Frequently asked questions

Does metformin cause weight loss?

Some people lose a small amount of weight, but it isn’t a weight-loss medicine.

Can I stop metformin if my sugar is normal?

Don’t stop without talking to your doctor; normal readings may be because the medicine is working.

For questions about a prescription or symptoms, speak with your own doctor or pharmacist. For website enquiries, contact our team.

This article is for general information only and isn’t a substitute for advice from a doctor or pharmacist.

Source

MedlinePlus: metformin patient information.