Biguanides

What it is

Biguanides are a drug class for type 2 diabetes; today metformin is the only widely used biguanide. The key practical fact is that metformin was approved in the United States in 1994 and is now the only biguanide in routine clinical use because older agents in the class, such as phenformin and buformin, were withdrawn or largely abandoned due to a higher risk of lactic acidosis.

Biguanides lower blood glucose without directly stimulating the pancreas to release insulin. That matters because, when used alone, metformin usually has a low risk of causing hypoglycemia. It is commonly used as first-line treatment for type 2 diabetes alongside diet, physical activity, and weight management.

Within this class, the real-world comparison is simple:

Drug	Current status	Main point
Metformin	Widely used	Standard biguanide for type 2 diabetes
Phenformin	Withdrawn in many countries	Higher lactic acidosis risk
Buformin	Not in routine use in many settings	Similar safety concerns limited use

Metformin is available in immediate-release and extended-release forms and is also included in many fixed-dose combinations with other diabetes medicines. In India, it is widely prescribed for type 2 diabetes and is familiar in both primary care and specialist diabetes practice.

How it works

Biguanides mainly work by reducing glucose production in the liver. Metformin also improves insulin sensitivity, especially in the liver and peripheral tissues, and decreases intestinal absorption of glucose to a smaller extent. The overall effect is lower fasting blood glucose and, in many patients, a modest improvement in HbA1c.

At the cellular level, metformin affects energy metabolism, including mitochondrial pathways and AMP-activated protein kinase signaling, although not every clinical effect is explained by a single mechanism. It does not act like sulfonylureas, which increase insulin secretion. Because it does not force the pancreas to release more insulin, metformin alone is less likely to cause low blood sugar.

A few practical points help explain why clinicians often choose this class first:

1. It lowers glucose effectively, especially fasting glucose.
2. It is generally weight-neutral or may cause modest weight loss in some people.
3. It has a long track record and is available as a low-cost generic.
4. It can be combined with most other diabetes drug classes, including insulin.

Evidence and uses

The main evidence-based use of biguanides is type 2 diabetes, and in current practice that means metformin. Major guidelines have long supported metformin as an initial medicine for many adults with type 2 diabetes, unless there are contraindications such as advanced kidney dysfunction or intolerance.

Common clinical uses include:

Use	Evidence status	Notes
Type 2 diabetes	Strong	Standard first-line or add-on therapy
Prediabetes in selected high-risk people	Guideline-supported	Used when lifestyle measures alone may not be enough
Polycystic ovary syndrome (PCOS)	Common off-label use	May improve insulin resistance and menstrual patterns in some patients
Gestational diabetes	Selected use	Used in some settings, but treatment choice depends on clinician judgment and local guidance
Antipsychotic-associated weight gain	Off-label	Some evidence of benefit in selected patients

For type 2 diabetes, metformin has the strongest support for lowering blood glucose and HbA1c. It is often the first oral medicine tried after diagnosis, especially when lifestyle changes alone do not achieve glucose targets. It is also commonly continued when other medicines are added later.

For prediabetes, evidence supports metformin in some higher-risk groups, but lifestyle intervention remains the foundation. For PCOS, metformin may help with insulin resistance and ovulation-related outcomes in some people, though it is not a universal solution. For possible anti-aging, anticancer, or neuroprotective effects, research is ongoing, but these are not established routine indications.

Safety and interactions

The main side effects of biguanides, specifically metformin, are gastrointestinal. Nausea, diarrhea, abdominal discomfort, and reduced appetite are common, especially when treatment is started or the dose is increased. Taking it with food and using extended-release formulations can improve tolerability.

Important safety issues include:

1. Kidney function: Metformin is not used the same way in all levels of kidney function. Clinicians usually check estimated glomerular filtration rate, or eGFR, before starting and during treatment.
2. Lactic acidosis: This is rare but serious. Risk rises in settings such as severe kidney impairment, severe illness with low oxygen states, sepsis, dehydration, or major liver disease.
3. Vitamin B12 deficiency: Long-term metformin use can reduce vitamin B12 levels in some patients. This can matter if a person develops anemia, numbness, tingling, or worsening neuropathy.
4. Temporary interruption in acute illness or procedures: Metformin may need to be held around some contrast imaging studies or during acute dehydration, severe infection, or surgery, depending on kidney function and clinical status.

Drug interactions are fewer than with many other diabetes medicines, but they still matter. Alcohol excess can increase the risk of lactic acidosis. Some drugs can affect kidney function or metformin handling, which may increase side effects. Patients should tell their clinician or pharmacist about all prescription drugs, over-the-counter medicines, and supplements.

Metformin does not usually cause hypoglycemia by itself, but low blood sugar can happen when it is combined with insulin or insulin-releasing drugs. People with diabetes should know the symptoms of hypoglycemia if they use combination therapy.

When to see a clinician

See a clinician if blood sugar remains high despite treatment, if side effects are persistent, or if symptoms suggest complications. Urgent review is needed for severe vomiting, dehydration, trouble breathing, unusual sleepiness, severe weakness, or confusion, especially during illness, because these can signal a serious metabolic problem.

Patients taking metformin should also seek review if they develop symptoms that could suggest vitamin B12 deficiency, such as fatigue, pale skin, numbness, burning feet, or balance problems. Regular follow-up usually includes HbA1c, kidney function, and sometimes vitamin B12 testing during long-term use.

In India, where type 2 diabetes is common and often appears at younger ages and lower body mass index than in some Western populations, regular follow-up is especially important. Diet pattern, kidney health, affordability, and access to monitoring all affect how metformin is used in practice.

Limitations and open questions

Although biguanides are a well-established class, the class is effectively represented by one medicine in modern practice: metformin. That means many statements about biguanides are really statements about metformin rather than multiple currently used drugs.

Metformin is effective, but it is not enough for everyone. Some patients need additional medicines early because of marked hyperglycemia, cardiovascular disease, chronic kidney disease, or obesity-related treatment goals. Also, not all patients tolerate it because of gastrointestinal side effects.

There are still open questions about its full mechanism, the best way to predict who will respond most strongly, and whether it has clinically meaningful benefits beyond glucose lowering. Evidence for anti-aging, cancer prevention, and other non-diabetes uses remains limited or mixed in humans. Those possible uses should not be treated as established benefits.

The bottom line is that biguanides are an important diabetes drug class historically, but in current care the term mostly refers to metformin, the only biguanide still widely used because it balances effectiveness, cost, and safety better than older drugs in the same class.