Metformin is a prescription drug intended for patients that need to treat type 2 diabetes. It can be taken alone or in combination with other drugs as prescribed by your health care provider. This medicine is associated with a number of side effects that are possible even if you are taking metformin exactly as recommended. You will need to tell your doctor about any of the following serious side effects, as they are quite rare and need to be analyzed carefully before you can carry on with the treatment: extreme tiredness, nausea, shortness of breath, decreased appetite, weakness, muscle pain, lightheadedness, fast or slow heartbeat, vomiting, dizziness, and stomach pain. There is no need to worry if you get a few side effects that are considered to be mild - such as gas, muscle pain, flushing of the skin, cough, bloating, stomach pain, constipation, heartburn, diarrhea, metallic taste in mouth, runny nose, sneezing, or headache. It's very important for the success of your treatment that you take this medicine exactly as prescribed without skipping doses or missing them. Taking a double dose instead of the one you missed can cause the following symptoms of an overdose: extreme tiredness, nausea, muscle pain, discomfort, stomach pain, vomiting, lightheadedness, decreased appetite, weakness, and dizziness.