Metformin can cause side effects in susceptible people who use the medication. For example, metformin can cause fatigue, nausea, vomiting, weakness, muscle pain stomach pain, shortness of breath, a slow heartbeat and dizziness. Some people will also experience hypersensitivity or an allergic reaction to metformin when using it and should avoid metformin in these circumstances. A rare, but serious, side effect that can result from metformin use is lactic acidosis. People with kidney disease should not use metformin unless instructed by their doctor or medical provider. People should avoid drinking alcohol while using metformin. All people using metformin should discuss its use and possible side effects with their pharmacist and a licensed medical provider prior to use.