Metformin: Uses, dosage, side effects, warnings & patient reviews

What is Metformin?

Metformin is an oral prescription medication belonging to the biguanide class. It is commonly used as initial pharmacologic therapy for managing type 2 diabetes. It is widely recommended in clinical guidelines due to its established efficacy, safety profile, and affordability, making it a cornerstone of diabetes treatment.

Unlike some other diabetes medications, metformin does not stimulate insulin secretion from the pancreas. Its primary mechanisms of action are to decrease the amount of glucose produced by the liver (hepatic gluconeogenesis) and to improve the body’s sensitivity to insulin, particularly in muscle tissue. This helps lower fasting and post-meal blood sugar levels. A notable characteristic is that it generally does not cause hypoglycemia (low blood sugar) when used alone and may be associated with modest weight loss or weight neutrality.

Available Forms and Strengths

Metformin is available as immediate-release tablets, extended-release (ER or XR) tablets, and an oral solution. Common tablet strengths include metformin 500 mg, metformin 850 mg, and metformin 1000 mg. The extended-release formulation is designed to be taken once daily and may help reduce gastrointestinal side effects.

How to Take Metformin

Adherence to prescribed dosing is crucial for effectiveness and to minimize common gastrointestinal side effects.

Administration and Dosage Initiation

Metformin is typically started at a low dose to improve tolerance. A common starting dose is 500 mg once or twice daily, or 850 mg once daily with meals. The dose is gradually increased based on blood sugar response and tolerance, usually at intervals of one to two weeks. The maximum recommended daily dose is usually 2000 mg, with a maximum absolute dose of 2550 mg, divided into 2-3 doses for the immediate-release form.

Swallow tablets whole with a full glass of water. Take metformin with meals to significantly reduce the risk of stomach upset, nausea, or diarrhea. Renal function and glycemic response should be monitored periodically, particularly during dose escalation and in patients at increased risk of adverse reactions.

Switching to Extended-Release

If gastrointestinal side effects are problematic with the immediate-release form, your doctor may switch you to metformin extended-release (ER). This formulation is taken once daily with the evening meal and can provide better tolerability for some patients.

Course of Treatment

Metformin is a long-term medication for managing type 2 diabetes. It is essential to take it consistently as prescribed, even when you feel well, to maintain stable blood glucose control. Do not stop taking metformin without consulting your doctor.

Missed Dose

If you miss a dose, take it as soon as you remember with food. If it is almost time for your next dose, skip the missed dose and resume your regular schedule. Do not take a double dose to make up for a missed one.

Side Effects of Metformin

Most adverse reactions associated with metformin are gastrointestinal in nature and are dose-related, particularly during treatment initiation. For a broader understanding of medication side effects, you can refer to our realistic guide to side effects.

What is Metformin Used For?

Metformin is primarily used for the management of hyperglycemia (high blood sugar) in type 2 diabetes.

Primary Medical Use

First-line treatment for Type 2 Diabetes Mellitus: Metformin is recommended by major clinical guidelines as initial pharmacologic therapy for type 2 diabetes, often in conjunction with lifestyle modifications (diet and exercise). It effectively lowers both fasting and postprandial (after-meal) blood glucose levels. Good glycemic control with metformin helps prevent long-term diabetes complications.

Other Approved and Investigational Uses

• Polycystic Ovary Syndrome (PCOS): This is an off-label use. Metformin may be prescribed to address insulin resistance and menstrual irregularities in selected patients.
• Prediabetes: In certain high-risk individuals, metformin may be considered to delay progression to type 2 diabetes, in accordance with clinical guidelines.
• Weight Management: While not a weight-loss drug, the modest weight loss or weight-neutral effect associated with metformin can be beneficial for overweight or obese patients with diabetes.

Combination Therapy

If metformin alone does not achieve adequate blood sugar control, it is often combined with other antidiabetic agents. A common and convenient option is a fixed-dose combination pill, such as Janumet, which contains metformin and sitagliptin. For patients needing significant weight loss and glycemic control, newer agents like Rybelsus (semaglutide) may be added to or used after metformin.

Important Safety Concerns and Considerations

While metformin is a safe drug for most, certain risks require awareness and monitoring.

Lactic Acidosis Risk

Lactic acidosis is a rare but serious metabolic complication. The risk is increased in conditions that can lead to metformin accumulation or increased lactate production.

Key risk factors include:

• Severe Kidney Impairment: Metformin is excreted by the kidneys. Impaired kidney function can lead to drug accumulation. Its use is contraindicated in patients with an eGFR below 30 mL/min and requires caution and dose adjustment in those with an eGFR between 30-45 mL/min. Learn more about kidney disease.
• Acute conditions that can affect kidney function or lactate production, such as severe infection, dehydration, liver disease, heart failure, or excessive alcohol intake.

Symptoms include profound weakness, unusual muscle pain, trouble breathing, stomach discomfort with nausea/vomiting, feeling cold, dizziness, and slow heart rate. This condition is potentially life-threatening and requires immediate medical attention.

Vitamin B12 Deficiency

Long-term use of metformin (typically over several years) can interfere with the absorption of vitamin B12, potentially leading to deficiency. This can cause anemia and neurological symptoms like numbness or tingling in the hands and feet. Periodic monitoring of vitamin B12 levels is recommended for patients on long-term therapy.

Contrast Imaging Procedures

Metformin should be temporarily discontinued before any imaging procedure that uses iodinated contrast dye (e.g., CT scan, angiogram). The dye can cause acute kidney injury, increasing the risk of lactic acidosis. Your doctor will provide specific instructions on when to stop and restart the medication.

Critical Warnings for Metformin Use

Contraindication in Renal Impairment

Metformin is contraindicated in patients with severe renal impairment (eGFR <30 mL/min) due to the high risk of lactic acidosis. Its use in moderate renal impairment (eGFR 30-45 mL/min) requires careful assessment, potential dose reduction, and frequent monitoring of kidney function.

Avoid Use in Metabolic Acidosis

Metformin should not be initiated in patients with existing metabolic acidosis, including diabetic ketoacidosis (DKA). Diabetic ketoacidosis should be treated with insulin.

Hepatic Disease Caution

Metformin should be used with caution in patients with liver disease because impaired liver function can affect the body’s ability to clear lactate, potentially contributing to lactic acidosis.

Cardiovascular Safety

Metformin has demonstrated cardiovascular safety and may provide cardiovascular benefit in certain patient populations with type 2 diabetes. It does not carry specific cardiovascular risk warnings associated with some other antidiabetic agents. Effectively managing diabetes with metformin is part of a strategy to reduce long-term heart disease risk.

Surgical Procedures

Metformin should be withheld for 48 hours prior to elective surgical procedures and can be restarted only after normal renal function is confirmed post-operatively. This is due to the risk of altered renal function during surgery.

Metformin Dosage Information

Dosage is individualized based on efficacy and tolerability. The following table provides general guidelines.

Important Administration Notes

• Take with Food: Always take metformin tablets with a meal to minimize gastrointestinal side effects.
• Do Not Crush or Chew ER Tablets: Extended-release tablets must be swallowed whole.
• Pediatric Dosing (≥10 years): For IR tablets, the starting dose is typically 500 mg twice daily, with a maximum of 2000 mg per day.
• Renal Impairment: Initiation of metformin is generally not recommended when eGFR is below 45 mL/min. It is contraindicated if eGFR is below 30 mL/min.
• Geriatric Dosing: Should not be titrated to the maximum dose; use the minimum effective dose with careful renal monitoring.

Drug Interactions with Metformin

Several medications can interact with metformin, affecting its levels or increasing the risk of side effects.

Additional Information

Cost and Generic Status

Metformin is a generic medication and is one of the most affordable antidiabetic drugs available. Its low cost is a significant advantage, making it accessible for long-term therapy. As with all generic medicines, it provides the same active ingredient and efficacy as the brand-name version at a lower price.

Historical and Clinical Significance

Metformin’s origins trace back to the plant Galega officinalis (French lilac). It was introduced for diabetes treatment in Europe in the 1950s and approved in the United States in 1995. It revolutionized type 2 diabetes management by providing an effective, weight-neutral therapy without causing hypoglycemia, solidifying its position as the foundational oral medication for this condition.

Reporting Side Effects

You are encouraged to report negative side effects of prescription drugs to the relevant national health authority (e.g., the FDA in the United States via the MedWatch program). Reporting helps improve overall drug safety monitoring.

References and Medical Sources

The information on this page is compiled from reputable medical sources and guidelines to ensure accuracy and reliability.

1. American Diabetes Association. “Pharmacologic Approaches to Glycemic Treatment: Standards of Medical Care in Diabetes—2023.” Diabetes Care. 2023.
2. U.S. Food and Drug Administration (FDA). Guidance on Metformin Hydrochloride. [Updated 2022]. https://www.accessdata.fda.gov/drugsatfda_docs/psg/Metformin%20Hydrochloride_final_Oral%20tab%20ER_RLD%2021202_RC05-08.pdf
3. MedlinePlus. Metformin. U.S. National Library of Medicine; [Updated 2022]. https://medlineplus.gov/druginfo/meds/a696005.html
4. Drugs.com. Metformin Professional Monograph. Drugs.com; [Updated 2024]. https://www.drugs.com/monograph/metformin.html

Disclaimer: The information on this site is provided for informational purposes only and is not medical advice. It does not replace professional medical consultation, diagnosis, or treatment. Do not self-medicate based on the information presented on this site. Always consult with a doctor or other qualified healthcare professional before making any decisions about your health.