Sounds like 'met-FOR-min'

Easy-to-read medicine information about metformin – what it is, how to take metformin safely and possible side effects.

Type of medicine Also called
  • Medicine to treat type 2 diabetes
  • Belongs to a group of medicines called biguanides

  • Metformin 

What is metformin?

  • Metformin is in a class of medications called biguanides and is used to treat type 2 diabetes.  
  • It works by decreasing the amount of glucose you absorb from your food, decreasing the amount of glucose made by your liver, increasing your body's response to insulin (a natural substance that controls the amount of glucose in the blood) and  increasing the use of sugar by your muscles.
  • It can be used alone or with other medicines, including insulin, to treat diabetes.
  • Metformin is also used for polycystic ovarian syndrome under specialist care as this is an unapproved indication (read more NZ Formulary)


  • The usual dose for adults with diabetes starts with 500 mg (1 tablet) 1–2 times daily, increased gradually as tolerated to 1.5–2 g daily in divided doses.
  • The maximum dose is usually 3 g daily in divided doses.
  • Your doctor will usually start you on a low dose and increase the dose gradually, depending on your blood sugar level. This allows your body to get used to the medicine and reduces unwanted side effects.
  • You may be asked to record your blood sugar levels over this time so your doctor can see how well the metformin is working.
  • Always take your metformin exactly as your doctor has told you. The pharmacy label on your medicine will tell you how much metformin to take, how often to take it, and any special instructions. Ask your pharmacist if you have any questions relating to your medication.
  • Treatment for diabetes is usually lifelong. Keep taking metformin everyday to control your diabetes. Speak to your doctor, nurse or pharmacist before stopping.

How to take metformin

  • Always take metformin with food, during a meal or just after a snack.
  • Take metformin at the same times each day.
  • Do not skip meals while taking metformin.
  • Limit or avoid alcohol while taking metformin. It affects the control of your blood sugar and increases your chance of getting side effects.
  • If you forget to take your dose, take it (with food) as soon as you remember that day.
  • But, if it is nearly time for your next dose, just take the next dose at the right time. Do not take double the dose.
  • Talk to your pharmacist if you have any questions regarding your medication.

Special instructions

  • Sometimes (very rarely) metformin may lower your blood sugar too much - called hypoglycaemia. It is likely to occur if you delay or miss a meal or snack, drink alcohol, exercise more than usual, or cannot eat because of nausea or vomiting.
  • This may cause you to feel weak, faint, dizzy or irritable. You may get a headache, tremor (shakes) or blurred vision.
  • If this occurs, drink something sweet such as a small glass of sweetened soft drink, or fruit juice or eat something sweet like lollies. Follow this up with a snack such as a sandwich. Tell your doctor or nurse if this happens. 
  • Sometimes metformin may build up in your body, and cause lactic acidosis. This is very rare. You are at highest risk if you are over 80  years of age, have kidney problems or heart failure. To avoid this, your doctor will monitor how well your kidneys are working and adjust your metformin dose accordingly. Also, your doctor may take you off metformin for a short time if you become dehydrated or experience severe diarrhoea, have a severe infection or are undergoing surgery or radiotherapy.  

Possible side effects

Like all medicines, metformin can cause unwanted side effects, although not everyone gets them. Often unwanted side effects improve as your body gets used to the new medicine.

Side effects What should I do?

  • Feeling sick (nausea) or vomiting
  • Diarrhoea

  • Take your metformin with food.
  • These are quite common when you first start taking metformin and usually go away with time.
  • Tell your doctor or pharmacist if troublesome.

  • Signs of an allergic reaction such as skin rashes, itching, redness or swelling of the face, lips, mouth or have problems breathing.
  • Tell your doctor immediately or ring Healthline on 0800 611 116
  • Signs of low blood sugar such as headache, dizziness, tremor, sweating, hunger, irritability, weakness, feeling shaky or anxious.
  • Drink something sweet such as a small glass of sweetened soft drink, or fruit juice or eat something sweet such as lollies.
  • Tell your doctor or pharmacist.
  • Signs of lactic acidosis such as feeling unwell, feeling sick (nausea), unusually tired, short of breath, fast breathing. 
  • Tell your doctor or pharmacist immediately or ring Healthline on 0800 611 116


Metformin may interact with a number of medications and herbal supplements so check with your doctor or pharmacist before starting metformin or before starting any new medicines.

Learn more

Medsafe Consumer Information Sheet: metformin

New Zealand Formulary Patient Information: metformin

Credits: Sandra Ponen, Pharmacist. Reviewed By: Dr Janine Bycroft, GP, Health Navigator NZ