Metformin Side Effects: What to Expect

Quick answer: Digestive side effects are the most common problem with metformin. They include nausea, loose stools, gas, a metallic taste and less appetite. For many people they ease within a few weeks, but not for everyone. This page can’t tell you to stop or change anything. That decision belongs to your prescriber.

You have a new prescription, or you started a few weeks ago and your stomach has other ideas. Maybe you’re running to the bathroom more than you’d like. Maybe food tastes strange. That’s a miserable way to start a long-term medicine. Here is what people commonly report, why it happens, what tends to make it worse, and which signs mean you should get medical help quickly. You’ll also find what to write down for your next appointment.

Why your gut reacts to metformin

Metformin works mainly by lowering the amount of sugar your liver releases and by helping your body respond better to insulin. But the gut is also involved. Metformin acts in the intestine, where it changes how sugar is handled and how some gut hormones behave. It also affects the bacteria that live there.

Researchers don’t fully agree on exactly why the stomach and bowels get upset. Several explanations are being studied. These include changes in gut hormones, bile acids and gut bacteria. What we can say is that the effect is real, common and well documented. It isn’t “in your head,” and it doesn’t mean you’re doing something wrong.

It also isn’t an allergy in most cases. It’s a known effect of how the medicine behaves in the digestive tract. Your body usually needs time to adjust, though how long varies from person to person.

What people actually report

Reports tend to fall into a few groups:

• Nausea, sometimes with a queasy, off feeling after eating
• Loose stools or diarrhea, sometimes urgent
• Gas, bloating and stomach cramps
• A metallic taste in the mouth
• Less appetite, or feeling full sooner than usual
• Indigestion or heartburn-type discomfort

Not everyone gets all of these. Some people get none. Others get one or two that are mild and fade. A smaller group find the effects hard to live with.

Appetite change deserves a quick note. Some people eat less on metformin and notice their weight shift a little. That varies a lot, and it isn’t a reason to take the medicine. If weight is on your mind, our page on metformin and weight loss goes into it more.

The first two weeks versus the long term

The early weeks are usually the roughest. Most digestive effects show up when you first start, or when your prescriber raises the amount you take. That’s when your gut is meeting the medicine for the first time.

For many people, symptoms ease over several weeks as the body adjusts. Others find they linger, come and go, or return after a change. There’s no promise either way, and anyone who gives you an exact timeline is guessing.

Longer term, many people take metformin without much trouble. Some still have occasional stomach upset. A few find it never really sits right.

Metformin can also affect how your body absorbs vitamin B12 over years of use. This is a known long-term effect, and it’s one reason prescribers sometimes check B12 levels. If you’ve taken it for a long time, it’s a fair thing to ask about. The American Diabetes Association mentions periodic B12 monitoring in its standards of care.

If you’re stuck in the “feel awful” stage, don’t just push through in silence. Tell your prescriber what’s happening.

What can make it worse

A few everyday things are commonly linked with more stomach trouble:

• Taking it on an empty stomach. Metformin is usually taken with meals. Your label and pharmacist will tell you how it should be taken for you.
• Very large meals. A huge plate on top of the medicine can leave your gut working overtime.
• Alcohol. Drinking can irritate the stomach. It also matters for a more serious reason, covered below. Our page on drinking beer with metformin explains this in more detail.
• Rich, greasy or very sugary food. Many people find these worsen loose stools, though it differs from person to person.
• A recent change. A newly raised amount, or a switch between versions, can bring symptoms back.

Noticing your own pattern is more useful than any general rule. A simple note of what you ate and when you took your medicine can show things you’d otherwise miss.

Slow-release versions: a question to ask

Metformin comes in more than one form. Besides the standard tablets, there are extended-release (often called slow-release or XR) versions. They release the medicine more gradually.

Some people find the slow-release type easier on the stomach. Others notice no difference. This page isn’t recommending it as a fix. Whether it suits you depends on your health, your kidney function and what your prescriber thinks is right.

What you can do is ask. Something like: “I’m having stomach trouble. Is there a different form of metformin that might be worth discussing?” That’s a normal question, and prescribers hear it often.

Please don’t switch forms, split tablets, skip doses or stop on your own. Those choices should go through the person who prescribed it. Changing things without guidance can leave you worse off and can muddy the picture of what’s actually helping.

Metformin is not insulin

A common worry is low blood sugar. Metformin on its own does not usually cause hypoglycemia (a low). It doesn’t push insulin out of your pancreas the way some other diabetes medicines do.

That said, lows can happen if metformin is combined with other glucose-lowering medicines, especially insulin or sulfonylureas. Heavy drinking, long gaps without food or a lot of exercise can also play a part. If you take more than one medicine, ask your prescriber which ones carry a risk of lows.

If you feel shaky, sweaty, confused or unusually hungry, check your blood sugar if you have a meter and tell your care team. Don’t assume it’s “just” the metformin.

Feeling unwell in the first weeks is often the gut, not your blood sugar. The two can feel alike, though, so it’s worth checking rather than guessing.

Rare serious signs: when to get help now

Serious problems are uncommon. But one is worth knowing about: lactic acidosis, a rare buildup of lactic acid in the blood. It’s more of a concern if your kidneys aren’t working well, if you drink heavily, or if you’re seriously ill or dehydrated. That’s why kidney function is checked when metformin is prescribed.

Get medical help straight away if you have:

• Trouble breathing, or fast, deep breathing that isn’t explained by exercise
• Extreme tiredness or weakness that is out of the ordinary for you
• Vomiting that won’t stop
• Severe stomach pain with muscle aches
• Feeling very cold, dizzy or faint, with an unusual sense that something is badly wrong

Call your doctor or local emergency number. Don’t wait for your next appointment.

This is rare. Most people on metformin will never face it. But if you have kidney disease, tell every clinician who treats you. The National Kidney Foundation and NIDDK both explain how kidney function affects medicine choices.

What to write down before your next appointment

A short log makes the visit far more productive. Try to note:

• The date you started, and any date your prescriber changed what you take
• Which symptoms you have, and how bad they are on an ordinary day
• When they happen relative to taking the medicine and eating
• What you ate on your worst days
• How often you’ve had loose stools or nausea
• Alcohol, if any, and when
• Other medicines and supplements you take
• Any blood sugar readings you’ve been taking
• Questions, including whether a slow-release form is worth discussing

Bring it on paper or on your phone. Be honest about missed doses. Prescribers would rather know than guess.

If you’re also tracking numbers, our A1C chart by age can help you make sense of your results before you talk them through with your care team.

Is it normal to feel sick on metformin? Yes. Nausea, loose stools and stomach upset are among the most commonly reported effects, especially early on. Many people find they ease with time, but not everyone does. If it’s hard to cope with, tell your prescriber. Don’t change what you take on your own.

Can I take metformin without food? Metformin is usually taken with meals, partly because it tends to be gentler on the stomach that way. Follow the directions on your label and check with your pharmacist or prescriber about your situation. If you’re unsure, ask before changing how you take it.

Will metformin give me low blood sugar? On its own, metformin doesn’t usually cause lows. The risk goes up if you also take insulin or certain other diabetes medicines, or if you drink heavily or go long periods without eating. Ask your prescriber whether any of your medicines can cause lows.

The bottom line

Stomach trouble is the most common complaint with metformin. It’s real, it’s usually worst early on, and for many people it settles. For some it doesn’t, and that’s worth telling your prescriber about. Slow-release versions exist, and asking about them is reasonable. Only your prescriber can decide whether to change, switch or stop. Learn the rare warning signs: trouble breathing, extreme tiredness and vomiting that won’t stop. If you notice them, get help right away. Keep a simple log, and take it with you to your appointment.

If weight is a concern, read about metformin and weight loss. If you enjoy a drink, see our guide on beer and metformin. And to make sense of your lab numbers, our A1C chart by age is a good place to start.

Sources: American Diabetes Association (diabetes.org), NIH/NIDDK (niddk.nih.gov), MedlinePlus (medlineplus.gov), National Kidney Foundation (kidney.org), CDC (cdc.gov).

This article is for general education and is not medical advice. Talk to your doctor, pharmacist or dietitian about your own care.

This page is education, not medical advice. Bring your questions and your own numbers to your doctor or care team — they know your history.

This guide is also available in Spanish: Metformina: efectos secundarios.

Educational content only — not a substitute for advice from your doctor or pharmacist. Always work with your care team.