Metformin is a first-line medication used to manage Type 2 diabetes and pre-diabetes. While it is highly effective at helping control blood sugar levels, many people experience side effects, especially when first starting treatment or increasing their dose. Most of these effects are related to the digestive system and tend to improve as the body adjusts.

The most common side effects include diarrhea, nausea, vomiting, gas, and bloating. Diarrhea and general stomach discomfort often appear early in treatment but usually improve within a few weeks. Nausea and vomiting can also occur at the beginning, and taking metformin with food may help reduce irritation. Gas and bloating are linked to changes in gut bacteria, and some people notice improvement after adjusting their diet or avoiding heavy, greasy foods during the adaptation period.

With long-term use, metformin may affect vitamin B12 absorption, potentially leading to deficiency. This can cause fatigue, anemia, or neurological symptoms such as tingling or numbness in the hands and feet. Doctors often recommend periodic blood tests for people on long-term therapy, and supplements may be needed if levels become low.

A rare but serious side effect is lactic acidosis, a dangerous buildup of lactic acid in the blood. Although uncommon, it is a medical emergency and requires immediate attention. Symptoms can include extreme weakness, muscle pain, difficulty breathing, and unusual tiredness. The risk is higher in people with kidney, liver, or heart problems, or those who consume excessive alcohol.