People with kidney disease (nephropathy) should not use biguanides such as metformin. These patients are at high risk of developing a buildup of lactic acid in the blood (lactic acidosis). This is the most serious side effect associated with metformin, and is potentially life-threatening.

A physician should test the kidney function of all candidates for metformin therapy. This can be done using a blood test called the serum creatinine, or a urine test.

Symptoms of lactic acidosis include weakness, fatigue and unusual muscle pain or stomach discomfort.

Liver disease, heart failure and alcohol abuse all can trigger lactic acidosis. Patients with a history of those conditions may not be good candidates for biguanide therapy. Alcohol affects how metformin is metabolized, so even patients who do not drink regularly should do so in moderation. Patients who drink to excess, even sporadic episodes of binge drinking, should alert their physicians before beginning therapy with metformin. Dehydration also raises the risk of lactic acidosis, so patients should talk to their physician about ways to keep hydrated in all circumstances.

Patients who are having surgery or medical tests that use dye should inform their physician of their use of metformin. The dyes can impair kidney function, causing the drug to build up in the blood. Patients may be advised to suspend their use of these medications for a period of time.

Other conditions that may affect use of metformin include:

• Severe injury, burns or infection
  
  
• Severe diarrhea or vomiting
  
  
• Female hormone changes that occur during puberty, menstruation or pregnancy
  
  
• Ketones (produced when fat is burned for energy instead of glucose) in the urine or blood
  
  
• Abnormal functioning of adrenal, pituitary or thyroid gland
  
  
• Severe intestinal problems, including slow stomach emptying
  
  
• Major surgery
  
  
• Poor food absorption and undernourishment
  
  
• Conditions that cause low glucose (blood sugar) or rapid glucose changes
  
  
• Heart or blood vessel disorders
  
  
• Liver disease
  
  
• Severe mental stress

Although metformin has not been associated with birth defects, the drug is rarely prescribed to women who are pregnant. Women should inform their physician if they are pregnant or planning to become pregnant before beginning treatment with metformin. Women who are breastfeeding should also notify their physician prior to beginning metformin treatment. Studies have not determined if the drug passes into a woman’s breast milk.