Metformin is one of the most commonly prescribed medications for type 2 diabetes and is also used for some people with prediabetes, insulin resistance, and other medical conditions. It has an excellent safety record and has helped millions of people manage their blood sugar.
Like any medication, however, metformin can cause side effects. The good news is that most side effects are mild, temporary, and improve as your body adjusts to the medication. Understanding what to expect—and knowing when to contact your healthcare provider—can make starting metformin much less stressful.
Why Does Metformin Cause Side Effects?
Metformin works primarily by reducing glucose production in the liver and improving the body's response to insulin. It also affects the digestive tract, which is why most side effects involve the stomach and intestines.
These digestive symptoms are especially common when starting metformin, increasing the dose, or taking the medication on an empty stomach. Fortunately, many people find that these symptoms improve within a few weeks.
The Most Common Side Effects
Digestive symptoms are by far the most common side effects of metformin:
- Nausea
- Diarrhea or loose stools
- Stomach discomfort
- Bloating and gas
- Mild abdominal cramping
- Loss of appetite
- A metallic taste in the mouth
Some people experience only one of these symptoms, while others notice several during the first few weeks.
How Long Do Side Effects Last?
For many people, digestive side effects begin shortly after starting metformin. The good news is that they usually improve as the body adapts — many patients notice significant improvement within one to four weeks.
If symptoms continue or become severe, contact your healthcare provider. Do not stop taking metformin without medical guidance unless you are experiencing a medical emergency.
How to Reduce Metformin Side Effects
Take it with food
Taking metformin during or immediately after a meal often reduces nausea and stomach upset. Avoid taking it on an empty stomach unless your healthcare provider specifically recommends otherwise.
Start with a low dose
Many healthcare providers begin with a lower dose and gradually increase it over several weeks, allowing your digestive system time to adjust. Never increase your dose faster than prescribed.
Ask about extended-release metformin
If digestive side effects continue, your healthcare provider may recommend extended-release (ER or XR) metformin. Extended-release formulations release medication more gradually and are often better tolerated than immediate-release tablets. For many people, switching formulations significantly improves symptoms.
Stay hydrated
Diarrhea can lead to dehydration. Drink plenty of fluids, especially if you're experiencing loose stools, and contact your healthcare provider if diarrhea is severe or persistent.
Give your body time
Many people stop metformin during the first week because they assume the side effects will continue indefinitely. In reality, digestive symptoms often improve substantially with time. If symptoms are mild, giving your body an opportunity to adjust may make a significant difference.
Can Metformin Cause Low Blood Sugar?
When taken by itself, metformin rarely causes hypoglycemia (low blood sugar) because it does not force the pancreas to release additional insulin. However, low blood sugar can occur if metformin is combined with medications such as insulin or sulfonylureas. Your healthcare provider will explain your individual risk based on your treatment plan.
Can Metformin Cause Weight Loss?
Some people lose a modest amount of weight after starting metformin; others experience no meaningful change. Metformin should not be considered a dedicated weight-loss medication, although improved insulin sensitivity and reduced appetite may contribute to modest weight loss for some individuals.
Vitamin B12 Deficiency
One important long-term consideration is vitamin B12 deficiency. Metformin can reduce vitamin B12 absorption over time. Although this doesn't happen to everyone, your healthcare provider may recommend periodic testing, especially if you have been taking metformin for several years.
Possible symptoms include fatigue, weakness, numbness or tingling in the hands or feet, difficulty with balance, memory changes, and anemia. If a deficiency develops, treatment is often straightforward with vitamin B12 supplementation — more in our Metformin and Vitamin B12 guide.
Rare but Serious Side Effects
Serious side effects from metformin are uncommon. One rare but potentially life-threatening condition is lactic acidosis. Today, lactic acidosis is very uncommon when metformin is prescribed appropriately and kidney function is monitored. The risk is higher in certain situations, including severe kidney dysfunction, severe dehydration, or serious medical illness. This is one reason your healthcare provider reviews your medical history and laboratory results before prescribing metformin.
When Might You Need to Temporarily Stop Metformin?
There are situations in which your healthcare provider may recommend temporarily stopping metformin: severe dehydration, persistent vomiting or diarrhea, certain imaging studies that use iodinated contrast dye (depending on your kidney function and the procedure), major surgery, or serious illness requiring hospitalization.
Do not stop metformin on your own unless directed by your healthcare provider. If you're unsure, contact your healthcare team for guidance.
When Side Effects Don't Improve
If digestive symptoms continue despite taking metformin with food and allowing time for adjustment, your healthcare provider may recommend a slower dose increase, switching to extended-release metformin, reducing the dose, evaluating for another cause of your symptoms, or considering a different medication if metformin remains difficult to tolerate. Many people who initially struggle with metformin are eventually able to take it successfully after these adjustments.
Frequently Asked Questions
Is diarrhea normal when starting metformin?
Yes. Diarrhea is one of the most common side effects and often improves during the first few weeks of treatment.
Should I stop taking metformin if it upsets my stomach?
Not necessarily. Many digestive symptoms improve over time. Before stopping metformin, contact your healthcare provider to discuss possible adjustments such as taking it with meals or switching to an extended-release formulation.
Is extended-release metformin better?
For many people, yes. Extended-release metformin often causes fewer digestive side effects while providing similar blood sugar benefits.
Does everyone get side effects?
No. Many people tolerate metformin with few or no noticeable side effects. Even among those who develop digestive symptoms, most improve as treatment continues.
When Should You Contact Your Healthcare Provider?
Contact your healthcare provider if side effects are severe or do not improve after several weeks, you cannot eat or drink because of nausea, diarrhea becomes persistent or causes dehydration, you develop symptoms that may suggest vitamin B12 deficiency, or you have questions about adjusting your dose or switching formulations.
Seek Immediate Medical Care If
Seek emergency medical care if you develop difficulty breathing; swelling of the face, lips, tongue, or throat; severe weakness or extreme fatigue with persistent vomiting; confusion or loss of consciousness; severe dehydration that prevents you from keeping fluids down; or chest pain or other symptoms of a medical emergency. Although serious reactions to metformin are uncommon, these symptoms require prompt medical evaluation.
The Bottom Line
Metformin is one of the safest and most extensively studied medications used to treat type 2 diabetes and reduce diabetes risk in selected people with prediabetes. While digestive side effects such as nausea and diarrhea are common when starting treatment, they are usually mild and improve as your body adjusts.
Taking metformin with food, starting at a low dose, increasing the dose gradually, and using an extended-release formulation when appropriate can make the medication much easier to tolerate. If side effects persist, don't stop the medication without speaking to your healthcare provider. Simple adjustments are often enough to improve tolerability while allowing you to continue receiving metformin's long-term health benefits.