Diarrhoea is the most common side effect of metformin. Here are things you need to know before starting metformin for type 2 diabetes.

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It is well known that metformin can cause diarrhoea and this is one of the most common side effect that people experience (The Prevalence of Chronic Diarrhea Among Diabetic Patients). It should ‘settle’ after starting on the medication. But what if it doesn’t? What does ‘settle’ actually mean and how do you know if you should continue taking it? Also, why should you actually take something which causes such symptoms?

What is metformin?

Metformin is a type of medication used to treat high blood glucose level in people with type 2 diabetes. It comes in the form of a pill with some brand names including Diabex, Glucophage, and Diaformin. It is often the first drug to be prescribed to people with diabetes. Metformin can also be used in women with gestational diabetes and polycystic ovary syndrome.

Metformin and Diabetes

Type 2 Diabetes and Metformin

When someone is diagnosed with prediabetes, some doctors may prescribe metformin. Type 2 diabetes occurs when an elevation in blood sugar levels occurs when a sugar called glucose can’t get into your fat and muscles where it is needed. Elevated blood glucose levels damages the lining of blood vessels leading to a range of complications in various organs of the body. For more information on type 2 diabetes, you can start reading the Type 2 Diabetes Overview page.

Prediabetes and Metformin

Prediabetes is a condition that describes a person who has a number of abnormal glucose blood tests suggesting they have borderline diabetes. It may also be called “impaired glucose tolerance” or “metabolic syndrome”. People with Prediabetes are very likely to progress to developing type 2 diabetes in the coming few years. More importantly, people with prediabetes are the most likely to recover and return back to normal by making a few important changes. For more information on prediabetes, you can start reading the Prediabetes Overview page.

Gestational diabetes and Metformin

Treating gestational diabetes can greatly improve pregnancy outcome, for both mother and child. For many women, they’re able to achieve normal blood glucose levels through diet alone, however some women may require medications. Metformin is now increasingly being used with success in the treatment of diabetes in pregnancy. For more information on gestational diabetes and diabetes in pregnancy, you can start reading the Gestational Diabetes Overview page.

Are there any other side effects of metformin?


Vitamin B12 deficiency

Another common side effect of metformin use is vitamin B12 deficiency. Vitamin B12 has many important functions in our bodies. It’s needed to help make new blood cells and help our brain and nervous system to function properly.

About 30% of people taking metformin experience a reduced ability to absorb B12 from the intestine and this results in about 5-10% having a B12 deficiency (Vitamin B12 deficiency and diabetes). A low B12 level can result in fatigue and other symptoms seen in anaemia but also causes peripheral neuropathy- pins and needles or numbness in the feet or hands. B12 levels can be checked via routine blood tests. B12 supplements are available by a small monthly injection over 3 months if levels are depleted as it is difficult to get enough in our diet to increase levels.

Metformin can interfere with the absorption process of vitamin B12, which can cause people with diabetes to become deficient in B12. If you haven't been tested for B12 deficiency, request a blood test next time you see your doctor. Treatment for vitamin B12 deficiency is super simple and includes either a intramuscular injection or oral supplementation. Your doctor will choose the best treatment option for you.

For more information, read the article on Vitamin B12 Deficiency and Diabetes.


Nasuea is another common gastrointestinal side effect of metformin. This affects about 7% of people but is reported to be higher if not taking a slow release formula (definitely worth checking out what you have been prescribed) (Metformin in Noninsulin‐Dependent Diabetes Mellitus). If it is continuing then it is not normal and will need to be discussed with your doctor. It is important to have this medication at mealtimes to minimize these effects.

Metallic taste

A few people complain of a metallic taste (up to 3%) (Metformin in Noninsulin‐Dependent Diabetes Mellitus) and this should resolve once your body gets used to the medication. If it continues talk to your doctor.


Some people have noticed an odour from the metformin tablets that is fishy or like a “locker room sweat”. Again, this is an unusual experience but If concerned speak to your pharmacist as a different brand can be given.

But why should I take metformin?

So, I guess by now you are wondering
why on earth you should take Metformin at all…

It is actually a really good medication that works not only to help regulate blood glucose levels but also decreases the amount of insulin you need. Slowing the actual progression of diabetes, it has also been linked to other health benefits such as reducing cardiovascular events such as heart attacks and strokes; lowering cholesterol levels and even protecting against cancers. For more information please refer to Benefits of Metformin.

Studies indicate that it should be the drug of first choice on diagnosis of type 2 diabetes for very good reasons (American Diabetes Association: Approaches to Glycemic Treatment). When someone is given the diagnosis of diabetes, metformin is the first oral drug to be prescribed alongside lifestyle interventions, including changes in diet and physical activity levels. It is also used in pre-diabetes, gestational diabetes and polycystic ovarian syndrome and is considered a safe medication to take with few side effects other than those affecting the gut.

It is important therefore to first weigh up the pros and cons of taking metformin and talk to your doctor before stopping it to ensure you are making the best choice for your diabetes and general health. It may be as simple as a reduction in dose or changing brands to make a real difference.

Talk to your doctor

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