Metformin (Biguanides)

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Metformin how does it work

Continuing with our medication series today we are discussing the class known as biguanides. You may this as Metformin which is what we will refer to it as from this point onwards. As with all the posts in this series, this is not advice about whether you should or should not take them. Only how they work and the pros and cons of each medication.

How does it work?

Metformin is an oral diabetes medication. It is used to treat both type 1 and type 2 diabetes. However, it is mostly used to treat type 2 diabetes. Metformin works by improving your body’s insulin sensitivity and thus reducing insulin resistance. In other words, it helps your insulin work better.

It also stops the liver releasing too much glucose. This helps to lower your baseline glucose production.

There are various types and brands but mostly they are just known as Metformin.

Why use Metformin?

Metformin usually acts as the first line medication in type 2 diabetes. There are 2 types. Normal and modified release. Modified release is used if you cannot tolerate regular Metformin. More on this in the ‘side effects’ section below.


Metformin is one of the oldest, cheapest and still most effective medications for type 2 diabetes treatment. The expected HbA1c reduction using it is between 0.8-2% (9-22mmol/mol).

Metformin also helps with weight loss and offers some cardiovascular benefit and protection. Particularly if commenced early after diagnosis.

It doesn’t cause low glucose levels either. Therefore, there shouldn’t be any need to monitor your glucose levels day to day. This can help lower the burden of having diabetes.

Metformin Dose

It comes in 500mg tablets and can be taken between once and 4 times per day. This will be gradually increased to ensure you can tolerate it.

Side effects of Metformin

Metformin’s leading side effect is gastrointestinal discomfort. Some people are unable to tolerate Metformin at all whereas others may be able to tolerate a lower dose. Therefore, you may not tolerate the maximum dose but could still get some benefit from using a lower dose.

If you cannot tolerate it at all, there is a slower releasing version that is usually better tolerated.

Taking the maximum dose requires having 4 tablets per day. If this is in addition to other medications it can quickly increase your medication burden. The pills are also quite large and therefore people with swallowing difficulties could struggle.

Long term use of Metformin can lower your B12 levels and cause deficiency.

It may also exacerbate the build up of lactate in the body. Therefore, any conditions where lactate may accumulate or the body becomes more acidic such as conditions like; diabetes keto acidosis, respiratory problems, alcohol related acidosis, dehydration amongst others, Metformin should be suspended.


As mentioned above, Metformin may be temporarily suspended with certain acute conditions where lactate accumulates in the body.

However, a more common contraindication is renal impairment. If this is an acute kidney injury Metformin may be suspended temporarily. If you suffer with long term renal impairment your medical team may advise you Metformin is no longer an option.

Alongside this Metformin may not be suitable with excessive alcohol intake. In any situations where you become dehydrated. Metformin might also be stopped if you have liver disease.

Need more help

If you need more support we offer a range of 1:1 consultancy services and online programmes focused on helping you improve your glucose control and health for the long term. These include our type 2 diabetes recovery programme, type 1 glucose stability programme and winning weight loss programme. Go to the pages using the links above or in the headers or get in touch if you have questions.