Metformin: Type 2 diabetes drug

Diabetes specialist nurse, Laurie van der Merwe, breaksdown the use, and side effects of metformin, a drug used to treat Type 2 diabetes.

What is metformin?

Metformin is a medication commonly used as a first-line treatment for Type 2 diabetes, particularly in overweight or obese people. It is also sometimes prescribed in people with pre-diabetes and women with polycystic ovarian syndrome (PCOS).

It has been in use for 60 years and has shown to improve blood glucose levels without causing hypoglycaemia or low blood glucose levels when used on its own. The drug doesn’t cause weight gain. In fact, it may contribute to weight loss and has been shown to reduce micro- and macro-vascular complications.

Metformin may also be sold under the brand names Glucophage, Mengen, Metored, Romidiab, Diaphage, Bigsens or Gluconorm.

How it works?

Metformin lowers blood glucose levels by improving the way the body uses insulin, particularly counteracting the effect of insulin resistance. It has no effect on the production of insulin from the beta cells of the pancreas. But rather increases how the muscles and fat use glucose and reduces the release of stored glucose from the liver. It also reduces the absorption of glucose in the gut and is excreted by the kidneys.

In woman with PCOS, metformin helps to lower insulin and blood glucose levels and can stimulate ovulation.

Who can and can’t take metformin?

Metformin may be prescribed for adults and children of 10 years and older. However, it may not be suitable if you have liver or heart failure and if you have serious gastro-intestinal disease.

Because metformin is excreted through the kidneys, if your kidneys are not functioning fully or the function of the kidneys has deteriorated your doctor may reduce your dose or stop it. It is safe to take before and during pregnancy as well as when breastfeeding.

Taking of metformin

Metformin is taken with water during or immediately after a meal. The tablets must be taken whole. It must never be crushed or broken in half as this disrupts the protective permeable polymer coating and may result in gastric side effects.

When starting metformin, it is advisable to start on a low dose of 500mg twice a day before increasing as prescribed by your doctor. The maximum dose of the medication should be 2000mg a day.

The medication comes in various dose tablets (500mg, 850mg and 1000mg) and is also available in a slow-release version to reduce the effect on the gastro-intestinal system.

Metformin has also been included in various fixed-dose combinations with other diabetes medications, such as in Jalra-Met, Galvus Met, Glucovance and Janumet. If your doctor has prescribed any of these medications for you please do not continue taking your metformin tablets.

If you forget to take a dose of metformin, just take your usual dosage when next required. Do not take both doses together.

Metformin, and many other medications, should never be taken with grapefruit whether fresh whole fruit, freshly squeezed or frozen as it can lead to a drug interaction with some chemicals found in grapefruit. These chemicals may inhibit an enzyme in the gut resulting in a larger amount of the medication being absorbed.

Possible side effects

As in most medications, metformin does have common side effects, although not everyone gets them. These may include nausea, vomiting or diarrhoea which usually disappear after a week or two. Another common side effect may be a metallic taste in the mouth.

Prolonged use of metformin can lead to a vitamin B12 and folate deficiency.

More serious side effects are rare, but if they do occur please call your doctor if you get these warning signs:

  • General especially abdominal discomfort, decreased appetite, changes in breathing.
  • Unusual severe tiredness, sleepiness or weakness.
  • Muscle pain or cramping.
  • Severe allergic reaction.
  • Yellowing of the skin or whites of the eyes.
  • Sore and red tongue, and mouth ulcers.
  • Muscle weakness and disturbed vision may be signs of vitamin B12 deficiency.
  • A skin rash or itching.
  • Low blood glucose when you take metformin with other diabetes medication.

How to treat side effects

Taking metformin with food or immediately after a meal will help reduce the side effects. If the side effects continue please discuss with your doctor. The metformin XR (extended release) has been associated with increased adherence due to less side effects, and to the slower rate of absorption and the protective permeable polymer membrane that covers the tablet.

Try chewing sugar-free gum to help reduce the metallic taste in the mouth.


There has been some discussion about metformin causing kidney damage. This is probably because when the kidneys are not filtering as well as they should, they are unable to excrete metformin so the dose may be reduced or discontinued.

One of the current discussion points is related to the withdrawal of certain slow-release metformin generics in USA (not available in SA) which were apparently shown to have possible contamination by NDMA, a nitrosamine product which they found had above intake limits and may increase the risk of cancer if the level is too high over an extended period.

However, the testing was done in a private laboratory and the FDA has retested and disputed their findings. A person taking a drug that contains at or below acceptable levels of NDMA taken every day for over 70 years show no additional risk.

Furthermore, there are indications of the opposite being true in relation to cancer with studies showing an improved overall survival for metformin users with colorectal cancers.

A further misconception is that metformin causes hair loss where in effect the hair loss is more probably caused by high blood glucose levels.

Please always speak to your doctor before stopping metformin or any diabetes medication.


Laurie van der Merwe is a registered nurse and is currently the Diabetes Specialist Nurse of the Empangeni Centre for Diabetes with a managed care and private practice. She is also a Diabetes Partner Coach for Guidepost.