Easy-to-read medicine information about metformin – what it is, how to take it safely and possible side effects.
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What is metformin?
Metformin is a tablet used to treat type 2 diabetes. It can be used alone or with other medicines, including insulin, along with good nutrition and regular exercise to treat diabetes. Metformin is also used in people with pre-diabetes to prevent type 2 diabetes. Read more about type 2 diabetes and pre-diabetes.
Metformin may be used to treat gestational diabetes (diabetes that develops during pregnancy). Read more about gestational diabetes and taking metformin for gestational diabetes (Māori and Samoan).
Metformin lowers your blood glucose levels by improving the way your body responds to insulin. Insulin is the hormone that controls the level of glucose in your blood. Metformin doesn’t cause weight gain and is commonly prescribed for overweight people with pre-diabetes.
Metformin is also used to treat polycystic ovarian syndrome.
In New Zealand metformin is available as tablets (500mg and 850 mg). Metformin is also available in combination with vildagliptin, called Galvumet.
- In New Zealand metformin is available as tablets (500 mg and 850 mg).
- The usual starting dose for adults with diabetes is 500 mg once or twice a day. Depending on your blood glucose levels, your doctor may increase your dose slowly over a few weeks. This allows your body to get used to the medicine and reduces side effects.
- The maximum dose is 3 grams daily (in divided doses) but for some people, such as older adults or people with kidney problems, the maximum daily dose should be lower.
- Each day's tablets are usually divided into two doses (breakfast and dinner), or sometimes as three doses (breakfast, lunch and dinner).
- 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.
My dose is:
How to take metformin
- Timing: Always take metformin with food – during a meal or just after a meal. Do not skip meals while taking metformin. Take metformin at the same times each day.
- Limit or avoid alcohol while taking metformin. It affects the control of your blood glucose and increases your chance of side effects.
- Missed dose: If you forget to take your dose, take it (with food) as soon as you remember. 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.
- Keep taking metformin regularly. To control your diabetes you must take metformin everyday. Treatment for diabetes is usually lifelong. Do not stop taking metformin without talking to your doctor first.
- Unwell from vomiting or diarrhoea (runny poos): If you are unwell and dehydrated from vomiting or diarrhoea, ask your healthcare provider for advice. They may recommend that you stop taking metformin until you are better.
Learn more: frequently asked questions about metformin.
Precautions – before taking metformin
- Do you have problems with your kidneys, liver or heart?
- Are you about to have surgery?
- Are you taking any other medicines? This includes any medicines you buy without a prescription, such as herbal and complementary medicines.
If so, it’s important that you tell your doctor or pharmacist before you start metformin. Sometimes a medicine isn’t suitable for a person with certain conditions, or it can only be used with extra care.
Diabetes and long-term metformin can both cause low levels of vitamin B12. You may need to have a blood test to check vitamin B12 if you have symptoms of anaemia (fatigue, dizziness, mouth ulcers), and may need to take a supplement for this.
Like all medicines, metformin can cause side effects, although not everyone gets them. Often side effects improve as your body gets used to the new medicine.
Low blood glucose (hypoglycaemia)
Sometimes (very rarely) metformin may lower your blood glucose too much. This is called hypoglycaemia. This is most likely to occur if you delay or miss a meal or snack, drink alcohol, exercise more than usual or can't eat because of nausea or vomiting.
Hypoglycaemia can occur if you are also taking other medicines for diabetes. Hypoglycaemia may cause you to feel weak, faint, dizzy or irritable. You may get a headache, tremor (shakes) or blurred vision. If this happens, 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.
Metformin and other medicines for diabetes can very rarely cause a condition called lactic acidosis. You are at highest risk if you have kidney problems, a severe infection, dehydration 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 having an x-ray where a dye is needed.
Contact your doctor immediately if you are being sick (vomiting) or feel very unwell, have muscle cramps or if you become unusually tired, or if you feel short of breath and your breathing becomes faster than normal – these are the signs of lactic acidosis.
Other side effects
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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.
The following links have more information on metformin.
- Metformin New Zealand Formulary
- Metformin - renal Impairment and risk of lactic acidosis Medsafe 2015
- Metformin safe prescribing SafeRx, Waitematā DHB