Metformin for PCOS: pros and cons, dosage, side effects and safety

Have you been prescribed metformin for PCOS? Are you curious about the pros and cons? Or are you looking to understand the dosage, side effects and safety concerns regarding metformin? Whether you are taking metformin or just curious this article dives into metformin for PCOS. Keep reading!

What is PCOS?

Polycystic Ovarian Syndrome or PCOS is a common endocrine or hormone disorder. There is no ‘typical’ person with PCOS, as it is so variable. Often the symptoms include menstrual cycle disturbances, acne, and unwanted hair growth, commonly on the face.

There is no cure for PCOS but medication like metformin may be prescribed to help with symptoms.

What is metformin?

Metformin is characterised as a biguanide drug and an oral antihyperglycemic agent (OHA). It is a prescribed medication to manage insulin resistance as it helps your body utilise the insulin your body is making better and also reduces the amount of blood glucose created by different mechanisms such as gluconeogenesis in the liver.

Therefore metformin has a licence in the UK for treating individuals with type 2 diabetes. However it does not have a licence for use in PCOS, so use in this way is described as ‘off-label’ and usually, it will be initiated by a specialist doctor.

What is insulin resistance?

One symptom and driver of PCOS is insulin resistance. Up to 80% of people with PCOS have insulin resistance, no matter their body weight or shape. Insulin resistance is the term used to describe your body’s inability to utilise insulin (a hormone) properly to allow your cells to absorb glucose from the blood.

Consequently, your body tries to overcome this by producing more insulin than it should. This means your pancreas, the organ secreting insulin, is working harder.

Symptoms of insulin resistance include increased thirst, frequent urination, increased hunger, blurred vision and headaches.

How does metformin work?

Metformin works as an insulin sensitiser and reduces the production of insulin in the body. Metformin lowers blood glucose and insulin levels by suppressing the liver’s production of glucose, increasing the sensitivity of the muscle cells, fat cells and liver cells to insulin and decreasing the absorption of carbohydrates consumed.

What are the pros of taking metformin for PCOS?

Despite being an “off-label” medication for PCOS, metformin has benefits for people with PCOS.

As mentioned metformin is a first-line pharmacological treatment for individuals diagnosed with type 2 diabetes mellitus because of its positive profile of its insulin-sensitising effects. In people with PCOS, metformin has been seen to reduce insulin resistance and lower blood glucose levels.

Metformin may also improve ovulation and menstrual cycle frequency in people with PCOS. Clinical studies have also demonstrated the benefits of metformin in reducing other symptoms of PCOS including hirsutism, infertility and minimising the risk of miscarriages in the first 12 weeks and gestational diabetes for those with PCOS.

>> Read More | PCOS and Fertility: the ultimate guide to conception

What are the cons of taking metformin for PCOS? 

Although metformin has the potential to provide many benefits, there are some cons to taking this medication.

GI side effects

The most common side effects of taking metformin are problems affecting the stomach and bowel, such as nausea and vomiting, diarrhoea, abdominal pain and loss of appetite. These side effects occur in more than 1 in 10 people taking metformin but they most often happen at the beginning of treatment.

The most common side effect is diarrhoea affecting 53.2% of people taking the drug.

If you have a history of functional gastrointestinal symptoms such as being diagnosed with irritable bowel syndrome then you can also discuss with your doctor the metformin-modified-release medication as an option which is usually better tolerated.

Potential for vitamin B12 deficiency

Most recently in May 2022, studies found that a common side-effect of a high dose and long-term prescription of metformin is vitamin B12 deficiency. A systematic review and meta-analysis looking at individuals with PCOS and individuals with type 2 diabetes found the higher the dose of metformin the more deficient individuals were in vitamin B12.

If you are taking metformin then blood tests should be used to monitor levels of vitamin B12. You will need B12 supplements or injections if you are deficient to prevent vitamin B12 deficiency anaemia.

4 ways you can help reduce side effects when taking metformin

  1. As directed by your doctor or specialist take the medication with meals to make sure you are not taking metformin on an empty stomach
  2. Ensure the meal you have is substantial, for example containing a source of carbohydrates, protein and vegetables to create an overall balance
  3. Be absolutely clear as to the amount of metformin you are taking daily and when
  4. If you have a history of gastrointestinal discomfort discuss the possibilities of trialling the metformin modified-release medication instead

It is also important to be aware of the risks of drinking alcohol with metformin.

FAQ: Metformin for PCOS

If you have questions about metformin for PCOS, we have answers!

What is the dosage of metformin for PCOS?

In the UK the dosage of metformin for PCOS usually begins between 250mg to 500mg per day and gradually builds up to 2g of metformin per day depending on the side effects experienced. The gradual increase of metformin taken daily usually occurs weekly until the goal dose is achieved and can vary from person to person.

How long should I take metformin for PCOS?

There is no time limit for taking metformin but if symptoms don’t improve after 6 months then your doctor may advise you to stop taking it.

Is there a natural alternative to metformin?

There is limited evidence comparing the efficacy of metformin to natural alternatives. However, one natural alternative that has promising evidence is inositol, an insulin sensitiser.

What are the side effects of metformin?

Metformin can cause GI issues like nausea, abdominal bloating and flatulence. More severe symptoms like vomiting or diarrhoea may also be present.

Who shouldn’t take metformin?

Metformin shouldn’t be used for PCOS if you have kidney dysfunction, congestive heart failure, impaired liver function or chronic or acute metabolic acidosis.

Can metformin be taken during pregnancy?

Metformin is safe in pregnancy and some evidence has shown that taking it up to 12 weeks of pregnancy may reduce the risk of miscarriages.

Key takeaways: Metformin for PCOS

Metformin is a commonly prescribed medication for PCOS, despite being “off-label”. It has various benefits for people with PCOS, primarily reducing insulin resistance. But, it also does have some cons specifically in relation to the side effects often seen in those taking the medication. As with all medication and supplements, speak to your healthcare team or doctor for advice and support regarding prescription medication.

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Sophia Boothby RD Author at The PCOS Collective

Lead Author | Head Dietitian | Registered Dietitian


Sophia is a Registered Dietitian working as a Specialist Community Dietitian within a London NHS Teaching Hospital specialising in gut health such as irritable bowel syndrome (IBS) and the low FODMAP diet, type 2 diabetes, PCOS, and cardiac rehabilitation. Sophia offers 1:1 PCOS support in our virtual PCOS clinic.

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