The most common side effects of metformin are digestive: diarrhea, nausea, vomiting, and gas. In clinical trials, diarrhea affected over half of participants, and nausea or vomiting affected about one in four. These symptoms are usually worst in the first few weeks and tend to ease as your body adjusts, especially if you take metformin with food and start at a low dose.
Digestive Side Effects
Stomach and bowel problems are by far the most reported side effects. In clinical data, diarrhea occurred in about 53% of users, nausea or vomiting in roughly 26%, and gas in about 12%. These numbers sound alarming, but they reflect what happens across large groups of people, and severity varies widely. Some people get mild loose stools for a week or two. Others deal with persistent diarrhea that disrupts daily life.
These symptoms are most likely to hit when you first start taking the medication or when your dose increases. Taking metformin with a meal makes a real difference. An empty stomach increases the chance of nausea and cramping. Most doctors start patients on a low dose and increase it gradually over several weeks, which gives the gut time to adapt. For many people, digestive side effects fade significantly within the first month or two.
There’s a common belief that the extended-release (ER) version of metformin is easier on the stomach than the immediate-release (IR) form. A systematic review comparing the two formulations found no statistically significant difference in rates of diarrhea, nausea, vomiting, abdominal pain, or gas. That said, individual experience varies, and some people do report feeling better after switching. If digestive symptoms are persistent, it’s a reasonable option to try.
Metallic Taste
Between 1 in 10 and 1 in 100 people on metformin notice a metallic or unpleasant taste in their mouth. It can make food taste slightly off or leave a lingering flavor that’s hard to describe. This side effect tends to be more noticeable early in treatment and often fades over time, though for some people it sticks around. It’s not harmful, but it can be annoying enough to affect appetite.
Vitamin B12 Deficiency
Long-term metformin use can lower your vitamin B12 levels. Updated safety information classifies this as a common side effect, potentially affecting up to 1 in 10 people taking the drug. The mechanism appears to involve multiple factors: metformin changes how the gut moves, can promote bacterial overgrowth, and reduces B12 absorption in the small intestine.
B12 deficiency develops slowly, often over years, and the symptoms can be subtle. Fatigue, tingling or numbness in the hands and feet, difficulty concentrating, and mood changes are typical signs. Because these overlap with symptoms of diabetes itself, B12 deficiency in metformin users often goes unrecognized. If you’ve been taking metformin for several years, periodic blood tests to check B12 levels are worthwhile, particularly if you’re experiencing unexplained nerve symptoms or fatigue. Supplementation with B12 can correct the deficiency.
Weight Changes
Unlike many diabetes medications, metformin doesn’t cause weight gain. In fact, most people lose a modest amount of weight. In the large Diabetes Prevention Program trial, participants on metformin lost an average of about 2.1 kg (roughly 4.5 pounds) over nearly three years, compared to almost no change in the placebo group. Over longer follow-up, the average settled around 2.5 kg of sustained weight loss.
About 30% of people in that trial lost more than 5% of their body weight in the first year. Those who responded well maintained a mean weight loss of 6.2% even after 15 years. So while metformin isn’t a weight loss drug, its tendency to promote mild weight reduction (or at least prevent gain) is generally considered a positive side effect rather than a problem.
Lactic Acidosis
The most serious potential complication of metformin is lactic acidosis, a dangerous buildup of lactic acid in the blood. This is extremely rare in people with normal kidney function. When it does occur, it’s almost always in the context of other serious health problems: severe kidney impairment, liver failure, heavy alcohol use, dehydration, or conditions that starve tissues of oxygen like heart failure or sepsis.
When metformin-associated lactic acidosis happens, it is life-threatening. In one intensive care study, about 19% of patients who developed this condition died. That’s why kidney function matters. People with severely reduced kidney filtration (below 15 mL/min) should not take metformin at all. Those with moderately reduced kidney function can often still use it safely at lower doses, but this requires monitoring.
For the vast majority of metformin users, lactic acidosis isn’t a practical concern. It’s worth knowing about so you can recognize the warning signs: unusual muscle pain, difficulty breathing, stomach pain, dizziness, and feeling unusually cold or weak. These symptoms together warrant immediate medical attention.
Effects on the Liver
Metformin does not harm the liver. In fact, research suggests the opposite. Studies have found that metformin appears to protect liver cells from damage caused by toxic substances and may reduce the risk of liver cancer in people with diabetes. It was once avoided in people with liver disease out of caution, but current evidence supports its safety and even potential benefit in many liver conditions.
One area where metformin hasn’t lived up to early hopes is non-alcoholic fatty liver disease. While it improves insulin resistance and body weight in these patients, systematic reviews have found it doesn’t significantly improve the actual liver tissue changes associated with fatty liver disease.
Managing Side Effects
Most metformin side effects are front-loaded, meaning they’re worst in the beginning and improve with time. A few practical strategies make a noticeable difference. Always take your dose with food, ideally with your largest meal if you’re on a once-daily schedule. Starting at the lowest effective dose and increasing slowly over two to four weeks gives your digestive system time to adjust. Staying well hydrated helps with both digestive symptoms and the rare risk of lactic acid buildup.
If side effects persist beyond the first couple of months, switching between immediate-release and extended-release formulations is a common next step, even though the clinical data on this is mixed. Some people simply tolerate one form better than the other. Splitting doses across meals rather than taking them all at once can also help. Persistent, severe digestive symptoms that don’t improve with these adjustments are worth discussing with your prescriber, as alternative medications are available.