Gastrointestinal side effects with bloating, gassiness or diarrhea are among the most common side effects of using medications such as metformin. Serious side effects include severe allergic reactions/anaphylaxis and lactic acidosis. Slowed heart rate, or bradycardia has been reported with use of metformin, however this reaction is very uncommon.
If you or someone you know has noted having a slowed heart rate, irregular heart rate, dizziness or fainting spells, this may or may not be related to metformin use. It is very important to see your health provider to evaluate why someone is having these symptoms.
Reference:
http://www.drugs.com/sfx/metformin-side-effects.html