Metformin Side Effects
It is possible that some side effects of metformin may not have been reported. These can be reported to the FDA here. Always consult a healthcare professional for medical advice.
For the Consumer
Applies to metformin: oral solution, oral tablet, oral tablet extended release
As well as its needed effects, metformin may cause unwanted side effects that require medical attention.
If any of the following side effects occur while taking metformin, check with your doctor immediately:More common
- Abdominal or stomach discomfort
- cough or hoarseness
- decreased appetite
- fast or shallow breathing
- fever or chills
- general feeling of discomfort
- lower back or side pain
- muscle pain or cramping
- painful or difficult urination
- blurred vision
- chest discomfort
- cold sweats
- cool, pale skin
- difficult or labored breathing
- fast, irregular, pounding, or racing heartbeat or pulse
- feeling of warmth
- increased hunger
- increased sweating
- redness of the face, neck, arms, and occasionally, upper chest
- shortness of breath
- slurred speech
- tightness in the chest
- unusual tiredness or weakness
- Behavior change similar to being drunk
- difficulty with concentrating
- lack or loss of strength
- restless sleep
- unusual sleepiness
Some metformin side effects may not need any medical attention. As your body gets used to the medicine these side effects may disappear. Your health care professional may be able to help you prevent or reduce these side effects, but do check with them if any of the following side effects continue, or if you are concerned about them:More common
- Acid or sour stomach
- excess air or gas in the stomach or intestines
- full feeling
- loss of appetite
- metallic taste in the mouth
- passing of gas
- stomach upset or pain
- weight loss
- Abnormal stools
- bad, unusual, or unpleasant (after) taste
- change in taste
- difficulty with moving
- discoloration of the fingernails or toenails
- flu-like symptoms
- joint pain
- runny nose
- stuffy nose
- swollen joints
For Healthcare Professionals
Applies to metformin: compounding powder, oral solution, oral tablet, oral tablet extended release
Metformin has been generally well tolerated. About 20% of patients experienced mild, transient side effects sometime during therapy.
Lactic acidosis is a medical emergency requiring immediate evaluation and treatment. The case fatality rate may be as high as 50.3%. Patients taking metformin who present with even vague medical illnesses such as myalgia, malaise, somnolence, abdominal discomfort, and so forth, should be evaluated for a metabolic etiology like lactic acidosis.
Signs and symptoms of severe acidosis may include vomiting, abdominal pain, nausea, dyspnea, hypothermia, hypotension, and bradycardia.
Laboratory evaluation of metformin-induced lactic acidosis generally includes determination of the following: blood glucose concentration, lactic acid concentration, serum electrolytes, blood pH, metformin concentration, and exclusion of ketoacidosis.
If lactic acidosis is present, immediate institution of general supportive care is indicated. Prompt hemodialysis is also generally recommended in order to correct the acidosis and remove metformin. Hemodialysis often results in rapid improvement. Some investigators have suggested that dialysis with a bicarbonate-buffered dialysate may be particularly effective.
Metabolic side effects have included lactic acidosis, which is a potentially fatal metabolic complication. The incidence of lactic acidosis has been about 1.5 cases per 10,000 patient years. The risk of lactic acidosis has been particularly high in patients with underlying renal insufficiency. Cases of lactic acidosis occurring in patients with normal renal function have been rarely reported. Concomitant cardiovascular or liver disease, sepsis, and hypoxia have also increased the risk of lactic acidosis.
Subnormal vitamin B12 levels have been reported, and may result in anemia or neuropathy. Serum vitamin B12 levels at 2 to 3 year intervals may be useful.
Hypoglycemia occurred uncommonly in patients receiving metformin as monotherapy. Strenuous exercise, decreased caloric intake, general debilitation, adrenal insufficiency, pituitary insufficiency, and ethanol use have increased the risk of hypoglycemia.
Gastrointestinal effects have included nausea, anorexia, metallic taste, diarrhea, dyspepsia, flatulence, and abdominal pain. One study has reported a 20% incidence of diarrhea.
Hematologic side effects have included malabsorption of vitamin B12, due to intrinsic factor deficiency and possibly other mechanisms, in as many as 30% of treated patients. Megaloblastic anemia has occurred. Discontinuation of metformin or supplementation with vitamin B12 has been necessary.
Other side effects include a single case of leukocytoclastic vasculitis with pneumonitis.
Hepatic side effects have included at least two cases of hepatitis associated with metformin use.
A 52-year-old female with a history of type II diabetes mellitus and hypertension developed lethargy, fatigue, and diarrhea after taking metformin for more than 2 weeks. After continuing metformin for a total of four weeks, her sclera became icteric and she was hospitalized. Aside from a soft systolic ejection fracture and a moderate degree of bilateral lower extremity edema, her physical examination was unremarkable. Laboratory data showed grossly elevated total bilirubin, AST, ALT, and alkaline phosphatase. Several days after her initial presentation all of her medications were discontinued. Her signs and symptoms significantly improved over the following several days, and she was discharged within two weeks of her hospitalization. The patient's presentation was considered consistent with drug-induced toxicity attributed to metformin.
Other side effects have included reports of a distinct drug odor. The odor is generally described as "fish or fishy". The odor report varies considerably between generic versions and seems to be less noticeable with film-coated formulations.
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