Metformin belongs to the class of medications called oral hypoglycemics, which are medications that lower blood sugar. It is used to control blood glucose (blood sugar) for people with type 2 diabetes. It is used when diet, exercise, and weight reduction have not been found to lower blood glucose well enough on their own.

Metformin works by reducing the amount of glucose made by the liver and by making it easier for glucose to enter into the tissues of the body. Metformin has been found to be especially useful in delaying problems associated with diabetes for overweight people with diabetes.

This medication may be available under multiple brand names and/or in several different forms. Any specific brand name of this medication may not be available in all of the forms or approved for all of the conditions discussed here. As well, some forms of this medication may not be used for all of the conditions discussed here.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

The recommended adult dose of metformin ranges from 500 mg 3 or 4 times a day to 850 mg 2 or 3 times a day. The maximum daily dose should not exceed 2,550 mg. Tablets should be taken with food whenever possible to reduce the risk of nausea and vomiting. Metformin may be used alone or with other medications that reduce blood sugar. To ensure that the medication is working well, monitor your blood glucose on a regular basis as directed by your doctor or diabetes educator.

Many things can affect the dose of medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor.

It is important to take this medication exactly as prescribed by your doctor. If you miss a dose, skip the missed dose and continue with your regular dosing schedule. Do not take a double dose to make up for a missed one. If you are not sure what to do after missing a dose, contact your doctor or pharmacist for advice.

Store this medication at room temperature, protect it from light and moisture, and keep it out of the reach of children.

500 mg
Each white, round, biconvex, film-coated tablet debossed on one side "RXP" on top of "500" and scored on the other side contains 500 mg of metformin HCl. Nonmedicinal ingredients: microcrystalline cellulose, sodium starch glycolate, polyvinylpyrrolidone, colloidal anhydrous silica, and magnesium stearate; film coating: lactose monohydrate, methylhydroxypropyl cellulose, titanium oxide, and polyethylene glycol.

850 mg
Each white, oblong, biconvex, film-coated tablet with both sides score notched, and debossed on one side "M" on one half and "850" on the other half, contains 850 mg of metformin HCl. Nonmedicinal ingredients: microcrystalline cellulose, sodium starch glycolate, polyvinylpyrrolidone, colloidal anhydrous silica, magnesium stearate; film coating: lactose monohydrate, methylhydroxypropyl cellulose, titanium oxide and polyethylene glycol.

Do not take this medication if you:

• are allergic to metformin or any ingredients of this medication
• are experiencing or recovering from severe infections, trauma, or surgery
• are pregnant or breast-feeding
• are suffering severe dehydration (have lost a lot of water from your body)
• are undergoing radiologic studies involving use of iodinated contrast materials
• drink large amounts of alcohol in the short term or on a regular basis
• have a history of lactic acidosis
• have acute/chronic metabolic acidosis (too much acid in the blood), diabetic ketoacidosis with or without coma, or history of ketoacidosis with or without coma
• have diseases associated with lack of oxygen to the tissues such as cardio-respiratory insufficiency
• have reduced kidney function
• have severe liver disease
• have type 1 diabetes (people with type 1 diabetes should always be using insulin)
• have very poor blood glucose control (these people should not take this medication as the only antidiabetic agent)

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

• diarrhea
• dizziness or lightheadedness
• increased sensitivity of skin to the sun
• loss of appetite
• metallic taste in mouth
• nausea
• passing of gas
• stomach ache
• vomiting
• weight loss

Although most of the side effects listed below don't happen very often, they could lead to serious problems if you do not check with your doctor or seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

• decreased coordination
• feeling cold
• feeling extremely weak, tired, or uncomfortable
• low blood sugar (mild), including:
• anxiety
• behavioural changes similar to being drunk
• blurred vision
• cold sweats
• confusion
• cool, pale skin
• difficulty concentrating
• drowsiness
• excessive hunger
• fast heartbeat
• headache
• nausea
• nervousness
• nightmares
• restless sleep
• shakiness
• slurred speech
• peripheral neuropathy (e.g., numbness, prickling, tingling in hands or feet)
• signs of anemia (low red blood cells; e.g., dizziness, pale skin, unusual tiredness or weakness, shortness of breath, rapid heartbeat, dark urine)
• slow or irregular heartbeat
• thinking difficulties (e.g., poor decision making, decreased concentration, twitching, trembling, trouble speaking or swallowing)
• trouble breathing
• unusual muscle pain

Stop taking the medication and seek immediate medical attention if any of the following occur:

• lactic acidosis (quick and severe), including:
• fast, shallow breathing
• muscle pain or cramping
• slow or irregular heartbeat
• unusual sleepiness
• unusual stomach ache (after the initial stomach ache that can occur at the start of therapy)
• unusual tiredness or weakness
• seizure
• signs of pancreatitis (e.g., abdominal pain on the upper left side, back pain, nausea, fever, chills, rapid heartbeat, swollen abdomen)

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Before you begin taking a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should take this medication. 

Alcohol intake: Anyone taking metformin should avoid excessive alcohol intake.

Blood sugar control: If you have fever, trauma, infection, or surgery, you may have a temporary loss of blood sugar control. At such times, your doctor may think it is necessary to stop metformin and temporarily inject insulin. Metformin may be started again after the problem is resolved.

Blood sugar monitoring: Monitor your blood sugar regularly at intervals as discussed with your doctor or diabetes educator.

Diabetes complications: The use of metformin (or any other medication used for diabetes) will not prevent the development of complications particular to diabetes mellitus (e.g., kidney disease, nerve disease, eye disease).

Diet: Metformin is a treatment to be taken in combination with a proper diet. Metformin is not a substitute for proper diet.

Dye or contrast agents: If you are going to have an X-ray procedure that uses dye or a contrast agent, you may need to stop taking this medication for a short time. Contact your doctor for instructions.

Kidney problems:  Kidney disease or reduced kidney function causes this medication to build up in the body, causing side effects. If you have kidney problems, your doctor should closely monitor your condition while you are taking metformin, as it may affect kidney function.

Lactic acidosis: Lactic acidosis is a rare but serious problem that occurs due to metformin accumulation (i.e., the body doesn't get rid of it fast enough) during treatment. If you have severe kidney disease you are at higher risk of developing lactic acidosis. Since alcohol may increase the risk of lactic acidosis, do not drink a lot of alcohol over the short- or long-term while taking this medication. When it does occur (very rarely), it is fatal in 50% of cases. There have been no reports of lactic acidosis in Canada when metformin was used as directed. If you experience symptoms of lactic acidosis (e.g., weakness, tiredness, drowsiness, unusual muscle pain, trouble breathing, stomach pain with nausea, vomiting or diarrhea, feeling cold, dizziness, light-headedness, or slow or irregular heartbeat), stop taking this medication and get immediate medical attention.

Liver function: Decreased liver function has been linked to lactic acidosis. This medication is not recommended for people with severely reduced liver function. If you have moderately reduced liver function, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Low blood sugar: Under usual circumstances, low blood sugar does not occur for people who take only metformin. Low blood sugar could occur when not enough food is eaten, especially when strenuous exercise is undertaken at the same time or when large amounts of alcohol have been consumed.

Reduced response: Over a period of time, you may become progressively less responsive to a particular treatment for diabetes because your diabetes worsens. If metformin fails to lower blood sugar to target levels, talk to your doctor. Your doctor may want to stop metformin or recommend another medication.

Surgery: This medication should be stopped temporarily for surgery (except for minor surgery where food and fluid intake is not restricted). You will be restarted on this medication once you are eating and drinking and your kidney function has been tested and is normal. Talk to your doctor for specific instructions.

Thyroid function: This medication may decrease thyroid hormone levels. Your doctor will monitor your thyroid levels with blood tests while you are taking this medication.

Vitamin B12 levels: This medication may decrease vitamin B12 levels. Your doctor will monitor your B12 levels with blood tests while you are taking this medication.

Pregnancy: This medication should not be used during pregnancy. If you become pregnant while taking this medication, contact your doctor immediately.

Breast-feeding: Metformin is believed to pass into breast milk. This medication should not be used if you are breast-feeding.

Children: The safety and effectiveness of this medication have not been established for children.

Seniors: Kidney function often decreases with age. As a result, seniors may be more likely to experience side effects of this medication.

There may be an interaction between metformin and any of the following:

• acetazolamide
• alcohol
• angiotensin converting enzyme inhibitors (ACEIs; captopril, enalapril, ramipril)
• anti-malarial medications (e.g., hydroxychloroquine, quinine)
• antipsychotics (e.g., cariprazine, clozapine, olanzapine, quetiapine, risperidone)
• beta adrenergic blockers (e.g., acebutolol, carvedilol, levobunolol, timolol)
• birth control pills
• bismuth subsalicylate
• buserelin
• cancer medications (e.g., belzutifan, bortezomib, vorinostat)
• cephalexin
• cimetidine
• corticosteroids (e.g., dexamethasone, prednisone)
• danazol
• diabetic drugs (e.g., gliclazide, insulin, linagliptin, lixisenatide, repaglinide)
• disopyramide
• diuretics (water pills; e.g., furosemide, hydrochlorothiazide)
• elagolix
• epinephrine
• estrogens (e.g., conjugated estrogen, estradiol, ethinyl estradiol)
• everolimus
• glucagon
• glycopyrrolate
• goserelin
• hepatitis C antivirals (e.g., glecaprevir, ledipasvir, voxilaprevir)
• HIV integrase inhibitors (e.g., bictegravir, dolutegravir)
• HIV protease inhibitors (e.g., atazanavir, darunavir, ritonavir)
• hydroxychloroquine
• iodinated contrast material
• lamotrigine
• leuprolide
• linezolid
• megestrol
• mifepristone
• monoamine oxidase inhibitors (MAOIs; e.g., moclobemide, phenelzine, rasagiline, selegiline, tranylcypromine)
• nicotinic acid (niacin)
• nonsteroidal anti-inflammatory drugs (NSAIDs; e.g., celecoxib, diclofenac, ibuprofen, ketorolac, naproxen)
• patiromer
• pegvisomant
• pentamidine
• progestins (e.g., dienogest, medroxyprogesterone, norethindrone)
• protein kinase inhibitors (e.g., ceritinib, nilotinib, sunitinib)
• quinolone antibiotics (e.g., ciprofloxacin, norfloxacin, ofloxacin)
• ranolazine
• selective serotonin reuptake inhibitors (SSRIs; e.g., citalopram, fluoxetine, paroxetine, sertraline)
• sirolimus
• somatostatin-like medications (e.g., lanreotide, octreotide, pasireotide)
• somatropin
• sulfamethoxazole
• tacrolimus
• terbutaline
• testosterone
• topiramate
• tramadol
• triptorelin
• trimethoprim
• verapamil
• warfarin

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

• stop taking one of the medications,
• change one of the medications to another,
• change how you are taking one or both of the medications, or
• leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.