What are the Long-Term Effects of Metformin?

Metformin is a prescription drug that is used to help control blood glucose levels in individuals with type 2 diabetes mellitus. It is commonly sold under the brand names Glucophage and Fortamet and is available in regular and slow-release tablets. Metformin works by acting on the liver and intestines to decrease secretion and absorption of glucose into the blood. It also increases the insulin sensitivity of muscles and tissues of the body so that they take up glucose more readily. MayoClinic.com underlines that as with any medication, metformin can cause unwanted side effects that may be common or more serious.


Patients taking metformin, particularly women may experience general malaise, fatigue, and occasional achiness. Malaise may be caused by other effects of metformin on the liver, kidneys, stomach and intestines, as noted by Drugs.com.

Digestive Effects

Gastrointestinal disturbances affecting the stomach and intestines are common effects of metformin. MayoClinic.com lists these to include nausea, vomiting and loose bowel movements or diarrhea. These symptoms may decrease by beginning with a low dose of metformin and gradually increasing it.

Vitamin B12 Malabsorption

Vitamin B12 malabsorption may also occur in some patients on metformin treatment. MayoClinic.com explains that a chemical in the stomach called intrinsic factor is required for the body to absorb vitamin B12. Metformin can interfere with this chemical, causing decreased absorption of the vitamin. Over the long term, a vitamin B12 deficiency can cause significant health risks as this essential vitamin is important for synthesis of DNA, red blood cell production and other biochemical functions in the body. Decreased vitamin B12 in the blood can lead to megoblastic anemia in which the bone marrow cannot adequately manufacture red blood cells. Though this type of anemia is not common, it can occur from long-term use of metformin, causing decreased vitamin B12 levels.

Organ Effects

Long-term metformin use can cause liver or kidney problems in some individuals, according to MayoClinic.com, because the medication alters liver function and is excreted by the kidneys. This is more common in patients that already have kidney or liver problems, in addition to type 2 diabetes. For this reason, organ function is assessed before patients begin metformin, and blood tests determine any abnormal changes in patients that are taking this medication.

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