Long-term use of metformin, the most widely prescribed medication for type 2 diabetes, can lead to a vitamin B12 deficiency, a common side effect that may cause nerve damage and anaemia, according to pharmacist Anil Sharma.
Sharma, a Committee Member and Regional Representative for the East of England at Community Pharmacy England, spoke to the Daily Mirror about the potential 'problematic' side effects for patients who have taken the drug for years. He described long-term users as those who have been on metformin for at least 4-5 years or longer.
How metformin works and who takes it
More than 150 million people worldwide take metformin daily, and in the UK, more than three million people are said to be taking the medication. It is primarily used as a first-line treatment to lower blood sugar levels in people with type 2 diabetes by reducing sugar production in the liver, decreasing sugar absorption in the intestines, and increasing the body's sensitivity to insulin.
Doctors may also prescribe metformin to manage insulin resistance and regulate menstrual cycles in people with Polycystic Ovary Syndrome (PCOS), to delay or prevent type 2 diabetes in high-risk people with prediabetes, and to control blood sugar during gestational diabetes. Because it manages chronic, long-term conditions rather than curing them, many people take it for years.
Long-term use and vitamin B12 absorption
Clinical evidence shows that metformin is generally safe for extended use, with no maximum timeframe, provided the kidneys remain healthy. The drug is cleared from the body by the kidneys, so doctors typically check kidney function to ensure it remains safe to continue.
However, Sharma said that taking metformin over several years can sometimes block the absorption of vitamin B12, leading to a deficiency, as it inhibits a specific transporter in the gut called THTR-2. According to Gov.uk, developing a vitamin B12 deficiency is a common side effect affecting up to one in 10 people. "The risk of low vitamin B12 levels increases with higher metformin dose, longer treatment duration, and in patients with risk factors for vitamin B12 deficiency," a statement reads.
Why B12 deficiency is a problem
Sharma said: "This is a problem since it can cause nerve damage (numbness, burning, or tingling in your hands and feet), anaemia (tired, weak, and dizzy), memory loss, confusion and depression." A vitamin B12 deficiency can lead to megaloblastic anaemia, which deprives organs of oxygen and causes chronic fatigue, weakness, and breathlessness. Over time, an unmanaged deficiency causes progressive, sometimes irreversible, nerve damage, resulting in persistent numbness, tingling in the hands and feet, muscle weakness, and loss of physical balance. It can also disrupt brain chemistry, potentially causing severe brain fog, memory loss, depression, or mood changes, alongside physical signs like a swollen, sore, red tongue and pale or yellowish skin.
Patients on metformin are advised to talk to their doctor if they notice new or worsening signs such as feeling very tired, a sore, red tongue, pins and needles, or pale or yellow skin. Blood tests may be needed to determine the cause. People can continue taking metformin while their vitamin B12 levels are being fixed, but should talk to their doctor before stopping treatment.
