Skip to main content

Metformin: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on May 23, 2022.

1. How it works

  • Metformin may be used to treat Type 2 diabetes.
  • Metformin increases the ability of cells to uptake glucose by improving their insulin sensitivity. It also works by decreasing the amount of glucose made by the liver and decreasing the absorption of glucose from the intestine.
  • Metformin belongs to the class of drugs known as biguanides.

2. Upsides

  • Metformin is the preferred therapy for people with Type 2 diabetes without any contraindications. It may be used in combination with other medications for Type 2 diabetes.
  • May be used off-label (not an FDA-approved use, but still a recognized use of the drug) in polycystic ovary syndrome. Metformin decreases androgen hormone levels, improves menstrual cycles, ovulation regularity, and insulin sensitivity.
  • Metformin rarely produces hypoglycemia (low blood sugar levels).
  • Metformin does not change how much insulin is secreted and it does not cause hyperinsulinemia (high insulin levels). Fasting insulin levels and the baseline insulin response may decrease.
  • Metformin has a unique mechanism of action and is like no other drug used for diabetes.
  • Less likely to increase body weight (in fact body weight may decrease while on metformin), in contrast to many other medications used for diabetes, including sulfonylureas and insulin.
  • May be used in children older than ten years.
  • Available in 500mg, 850mg, and 1000mg strengths.
  • Generic metformin is available.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • Diarrhea, nausea, flatulence, indigestion, headache, lack of energy, and taste disturbance are the most commonly reported side effects.
  • Rarely, metformin can cause lactic acidosis, a condition that can be fatal. The risk is higher in older people, those with kidney disease, taking other medications that may decrease kidney function, undergoing radiologic studies using contrast media, with liver disease, or in those with other medical conditions such as congestive heart failure.
  • May impair vitamin B12 absorption but rarely results in anemia. Trials of 29 weeks or longer in duration have shown low vitamin B12 levels occur in 7% of people.
  • Excessive alcohol intake should be avoided because it can potentiate the risk of lactic acidosis.
  • Should not be used in people with severe renal impairment. Metformin should not be initiated in people with an estimated glomerular filtration rate (eGFR) of less than 45 mL/min. The eGFR should be measured at least annually in people taking metformin.
  • Should not be used in people with liver disease, with acute or chronic metabolic acidosis, including diabetic ketoacidosis.
  • Metformin may need to be temporarily discontinued in people with restricted food or fluid intake because continued use may increase the likelihood of dehydration, low blood pressure, and kidney damage.
  • Weigh up the benefits versus risks before using metformin in pregnancy. Research has not determined a drug-associated risk for major birth defects or miscarriage, and there are risks to the mother and fetus associated with poorly controlled diabetes mellitus in pregnancy. metformin has also been used off-label as an ovulation induction agent for non-obese women with polycystic ovary syndrome. Limited data report that metformin is present in breastmilk but the effects on the breastfed infant are not known.

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

Metformin improves how the body responds to insulin and it is effective for the treatment of type 2 diabetes. Taste disturbances, diarrhea, and other stomach-related side effects are common side effects. It is unlikely to cause weight gain.

5. Tips

  • Take with a meal to reduce the risk of stomach upset. Once-daily dosages should be taken with the evening meal.
  • Metformin is usually started at a low dosage before being titrated up. Dosages may vary between individuals.
  • Splitting dosages throughout the day (rather than taking a single dose) may improve gastric side effects such as nausea, diarrhea, and indigestion.
  • Swallow slow-release tablets whole; do not crush, break, or chew.
  • Do not drink large amounts of alcohol or drink it daily while taking metformin because this may increase your risk of lactic acidosis.
  • Monitoring of blood sugars and other regular laboratory tests including kidney function are needed.
  • May need to be temporarily discontinued before undergoing investigations requiring contrast media, or if you become dehydrated.
  • The outer case of some slow-release metformin tablets may be visible in the stools; this does not mean the drug has not been absorbed.
  • Ensure you are educated about the importance of diet and exercise in the management of type 2 diabetes because it is important to use lifestyle measures to improve your condition in addition to medications.
  • Tell your doctor if you become unwell with severe vomiting, diarrhea, or a fever because these types of illnesses may lead to severe dehydration and you may need to temporarily stop metformin. Also, talk to your doctor if you develop cold hands or feet, muscle pain, dizziness, a slow heartbeat, have trouble breathing, stomach pains, or severe nausea or vomiting.
  • Talk to your doctor or pharmacist before taking any other medications, including those bought over-the-counter, to check that they will be compatible with metformin.
  • Tell your doctor if you are pregnant or intend to become pregnant while taking metformin. It is not known how metformin affects the fetus or a breastfed infant.

6. Response and effectiveness

  • The peak effects of metformin are seen within 2-3 hours with immediate-release forms or 4 to 8 hours with slow-release forms. May take up to 2 weeks of regular dosing for the maximum effects of metformin to develop.

7. Interactions

Medicines that interact with metformin may either decrease its effect, affect how long it works for, increase side effects, or have less of an effect when taken with metformin. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with metformin include:

  • antidepressants such as SSRIs (eg, fluoxetine, sertraline), monoamine oxidase inhibitors (MAOI) such as selegiline, isocarboxazid, and phenelzine
  • aspirin and other salicylates
  • calcium channel blockers
  • carbonic anhydrase inhibitors such as acetazolamide, dichlorphenamide, and methazolamide (all increase the risk of lactic acidosis developing)
  • cephalexin
  • cimetidine
  • corticosteroids
  • digoxin
  • dolutegravir
  • estrogens
  • fluoroquinolone antibiotics, such as ciprofloxacin or norfloxacin
  • insulin
  • iodinated contrast agents
  • isoniazid
  • lamotrigine
  • nicotinic acid
  • ondansetron
  • oral contraceptives
  • phenothiazines
  • phenytoin
  • procainamide
  • quinidine
  • sympathomimetics such as terbutaline or ritodrine
  • thiazide and thiazide-like diuretics, such as HCTZ
  • thyroid agents
  • topiramate
  • trimethoprim
  • vancomycin
  • vandetanib
  • verapamil
  • other medications that affect blood sugar levels or are used for diabetes, such as chlorpropamide, glimepiride, or glipizide.

Note that this list is not all-inclusive and includes only common medications that may interact with metformin. You should refer to the prescribing information for metformin for a complete list of interactions.


Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use metformin only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Copyright 1996-2023 Revision date: May 23, 2022.