Side effects associated with biguanides, such as metformin, often can be greatly reduced by taking the medication with food. Side effects may include:

• Stomach problems, including:
  
  
• Nausea and vomiting
• Diarrhea
• Abdominal bloating or gas
  
  
• Breathing difficulties
  
  
• Buildup of lactic acid in the blood (lactic acidosis)
  
  
• Resumption of ovulation in premenopausal women who are not ovulating. This might necessitate the use of contraception if pregnancy is not desirable.

Some patients may find that use of biguanides prevents the body from properly absorbing vitamin B12. This can increase the risk of B12 deficiency, which can cause nerve damage and other problems. A physician may suggest vitamin supplements as a way to restore proper levels of vitamin B12.

Biguanides can cause a significant increase in ovulation (up to eight times), which is one reason why they are used to treat polycystic ovarian syndrome. Women taking biguanides should consult with a physician, preferably an obstetrician-gynecologist (ObGyn), about the medication’s effect on fertility.