Biguanides for Type 2 Diabetes: How They Work

Biguanides are a class of blood glucose-lowering medicines used mainly to manage type 2 diabetes. Metformin is the best-known and most widely used biguanide; it helps the body use insulin more effectively and lowers the amount of glucose released by the liver.

Biguanides at a glance

Biguanides are medicines that improve blood glucose control, primarily in people with type 2 diabetes. The medicine most people mean when they refer to biguanides is metformin. It has been used for many years and remains an important treatment because it is effective, well studied, and usually does not cause low blood glucose when taken on its own.

Type 2 diabetes develops when the body does not respond to insulin as effectively as it should and, over time, may not make enough insulin to meet its needs. Biguanides address a central part of this process: they reduce the liver’s release of glucose into the bloodstream and help body tissues become more responsive to insulin. They do not replace insulin, and they do not cure diabetes, but they can support long-term glucose management alongside nutrition, movement, sleep, and other prescribed care.

Older biguanides, including phenformin, are no longer used in many countries because of safety concerns related to lactic acidosis. Metformin has a more favorable safety profile when it is prescribed appropriately and monitored. A clinician considers the person’s kidney function, overall health, other medicines, and treatment goals before recommending it.

How biguanides work in the body

After meals and between meals, the liver naturally makes and releases glucose to provide energy. In type 2 diabetes, this glucose production may be higher than the body needs. Metformin decreases this excess liver glucose output, which can lower fasting and between-meal blood glucose levels.

Biguanides also improve insulin sensitivity. This means that muscle, fat, and other tissues can respond better to the insulin already present in the body and take up glucose more effectively. Unlike some other diabetes medicines, metformin does not usually force the pancreas to produce more insulin. For this reason, hypoglycemia, also called low blood glucose, is uncommon when metformin is used alone.

Metformin may also have modest effects on intestinal glucose handling and appetite in some people. Weight often remains stable, and some people lose a small amount of weight while taking it. Individual responses vary, however, and it should not be viewed as a stand-alone weight-loss medicine unless a clinician has specifically prescribed it for an appropriate indication.

Who may use biguanides

Metformin is most commonly prescribed for adults with type 2 diabetes. It may be used soon after diagnosis, particularly when lifestyle measures alone are not enough to reach agreed blood glucose goals. It can also be combined with other diabetes medicines or insulin when a person needs additional glucose lowering.

In selected situations, clinicians may prescribe metformin for uses beyond type 2 diabetes. For example, it may be considered for some people with polycystic ovary syndrome (PCOS), especially when insulin resistance or impaired glucose regulation is present. Such use depends on the person’s symptoms, pregnancy plans, laboratory results, and local clinical guidance. It should always be individualized rather than started without medical assessment.

Biguanides are not appropriate for everyone. Significant reduction in kidney function can allow metformin to build up in the body, increasing the risk of complications. A clinician may avoid the medicine, reduce treatment, or pause it depending on kidney results and the person’s circumstances. Severe liver disease, heavy alcohol use, dehydration, serious infection, or conditions that reduce oxygen delivery to tissues also require careful review.

• Metformin may be used alone or alongside other diabetes treatments.
• It is generally not used as the main treatment for type 1 diabetes.
• Pregnancy, breastfeeding, surgery, and planned imaging tests should be discussed with the prescribing clinician.

Expected benefits and treatment goals

The main goal of biguanide treatment is to help keep blood glucose within a range agreed with the healthcare team. Improved glucose management can reduce the risk of diabetes-related complications over time, including damage affecting blood vessels, nerves, kidneys, and eyes. Results are assessed through home glucose measurements when appropriate, symptoms, and laboratory tests such as glycated hemoglobin, often called HbA1c.

Metformin is often valued because it has a low risk of causing hypoglycemia when used by itself. This benefit is especially relevant for people who drive, work shifts, exercise regularly, or have previously experienced low glucose episodes. However, the risk of hypoglycemia can increase if metformin is taken with insulin or medicines that increase insulin release. The treatment plan should therefore include clear advice on recognizing and treating low blood glucose if relevant.

Diabetes care is broader than a single prescription. Nutrition choices, regular physical activity, smoking cessation, sleep, blood pressure management, cholesterol control, and routine screening all contribute to health. People can learn more about the condition and its ongoing care through an individualized assessment for type 2 diabetes.

Side effects, safety and monitoring

The most common metformin side effects involve the digestive system. Nausea, loose stools, abdominal discomfort, bloating, or reduced appetite can occur, especially when treatment begins or the dose is increased. Taking the medicine with food, increasing it gradually when advised, or using an extended-release formulation may improve tolerance for some people. Persistent or troublesome symptoms should be discussed with a clinician rather than managed by stopping treatment independently.

Long-term use may be associated with lower vitamin B12 levels in some people. Vitamin B12 is important for healthy red blood cells and nerve function. A clinician may check levels periodically, particularly when treatment has been prolonged or when symptoms such as unusual tiredness, a sore tongue, numbness, tingling, or balance changes occur. These symptoms can have many causes, so assessment is important.

A rare but serious complication is lactic acidosis, a build-up of lactic acid in the blood. The risk is greater when metformin accumulates because kidney function is severely reduced or when the body is under major stress, such as severe dehydration, a serious infection, respiratory failure, shock, or severe heart or liver illness. Urgent medical assessment is needed for severe weakness, unusual sleepiness, persistent vomiting, abdominal pain, rapid or difficult breathing, or feeling very cold, especially during acute illness.

Healthcare professionals usually check kidney function before starting metformin and at intervals thereafter. People should tell every member of their healthcare team that they take metformin, including before surgery or certain imaging procedures using iodinated contrast dye. Temporary changes to the medicine may be recommended in specific situations to protect kidney function and support safe care.

Taking biguanides safely in daily life

Biguanides should be taken exactly as prescribed. A clinician may begin with a lower amount and adjust treatment gradually to reduce stomach-related side effects. Tablets are commonly taken with meals, but the correct timing and formulation depend on the prescription. Extended-release tablets should generally be swallowed whole unless a pharmacist or clinician confirms otherwise.

If a dose is missed, the safest approach is usually to take the next scheduled dose at the usual time rather than taking extra medicine. People should follow the instructions supplied with their prescription and ask a pharmacist for advice if they are uncertain. Taking more than prescribed does not improve glucose control and can increase the risk of adverse effects.

During vomiting, diarrhea, fever, poor fluid intake, or any illness that may cause dehydration, a person should contact a healthcare professional for sick-day guidance. The advice may include temporarily pausing certain medicines, including metformin, until eating, drinking, and kidney function are stable again. Alcohol should be discussed openly with the prescribing clinician because heavy alcohol use can increase the risk of lactic acidosis.

People should not assume that a normal home glucose result means follow-up is unnecessary. Regular appointments allow the care team to review HbA1c, kidney function, treatment tolerance, cardiovascular risk, and whether another medicine may be needed. Medication changes should be made with professional guidance.

When to seek medical care

Non-urgent medical advice is appropriate if digestive symptoms do not settle, interfere with daily life, or begin after a long period of stable treatment. A clinician should also be contacted about symptoms that could suggest vitamin B12 deficiency, such as persistent fatigue, tingling in the hands or feet, weakness, or changes in balance. These symptoms are not specific to metformin, but they deserve assessment.

Urgent medical care is needed for severe or persistent vomiting, severe diarrhea with signs of dehydration, marked weakness, confusion, fainting, chest pain, severe shortness of breath, or rapid and deep breathing. These symptoms may be caused by a serious illness and should not be attributed to a medicine without evaluation. People with diabetes should also seek prompt advice when blood glucose remains very high or very low despite their care plan.

Before planned surgery, hospitalization, or imaging involving contrast dye, the prescribing team should be informed about biguanide use. Acıbadem Health Point’s multidisciplinary specialists and JCI-accredited hospitals can assess and treat diabetes-related health needs for international patients, with care plans based on the individual’s medical history and current condition.

Frequently asked questions