While metformin is generally well tolerated, like all medications, it can have side effects.
In this article, we’ll look at how metformin works, its common, uncommon, and serious side effects, and how to alleviate them.
Key facts
- Metformin is a widely used medication for type 2 diabetes, but it can cause various side effects.
- Common side effects include gastrointestinal issues such as diarrhea, flatulence, and nausea. Some people may also experience a metallic taste after taking the medicine.
- Long-term use can lead to vitamin B12 deficiency and, in rare cases, lactic acidosis. It’s important to monitor for these conditions closely.
- Side effects can be managed by adjusting the medication’s dosage, switching to extended-release forms, or combining it with certain other medications. Some people, such as those with certain kidney or liver issues, should avoid metformin.
How metformin works
Metformin (brand names Glucophage, Glucophage XR, Fortamet, Glumetza, and Riomet, plus various generics) works by reducing the amount of sugar the liver releases throughout the day and increasing the body’s sensitivity to insulin.
It also reduces the amount of glucose (sugar) that is absorbed from the food you eat, which in turn reduces your blood sugar levels after eating.
Common side effects of metformin
The most common side effects of metformin include:
- Diarrhea (varying from mild to severe)
- Flatulence (passing gas)
- Nausea or vomiting
- Bloating and constipation
- Stomach discomfort
- Heartburn
- Headache
- Decreased appetite and weight loss
- Muscle pain or cramping
- A metallic taste in the mouth
Another common side effect of metformin is weight loss. Although the medicine is not FDA-approved for this use, it is frequently prescribed off-label for this purpose.
In the Diabetes Prevention Program (DPP) Study, average weight loss among metformin users was approximately 4.4 to 6.6 pounds over two to three years.
Uncommon (serious) side effects of metformin
Vitamin B12 deficiency
Recent research has shown that long-term use of metformin can result in vitamin B12 deficiency in some people. Symptoms of vitamin B12 deficiency can include:
- Anemia
- Inflammation of the tongue
- Fatigue
- Palpitations
- Pale skin
- Dementia
- Weight loss
- Infertility
Left untreated, B12 deficiency can cause significant nerve damage, ultimately leading to a diagnosis of peripheral neuropathy.
If a doctor isn’t aware of this risk, they could misdiagnose the cause of a person’s neuropathy as being the result of high blood sugar levels when it may in fact be a side effect of the B12 deficiency caused by metformin.
This nerve damage is irreversible, but further damage can be prevented by supplementing with a regular dose of vitamin B12.
Everyone taking metformin should have their B12 levels tested annually. In consultation with their healthcare provider, anyone with inadequate levels should start taking a B12 supplement immediately. (Vitamin B12 injections every few months may be recommended in some cases.)
Lactic acidosis
There have been a significant number of studies on metformin’s risk of causing lactic acidosis — a potentially fatal condition in which lactic acid builds up in the body. Symptoms of lactic acidosis can include:
- Nausea
- Vomiting
- Weakness
- Fatigue
- Fast, deep breathing
- Muscle cramps and body aches
Metformin comes with a “black box” warning about this risk, the most severe warning that the U.S. Food and Drug Administration (FDA) issues.
That said, a large-scale study reported in 2022 examined 4,241 cases of suspected metformin-associated lactic acidosis and found that only a small fraction (9 percent) had sufficient data to support such a diagnosis, underscoring the rarity of this condition.
Almost all cases of lactic acidosis from metformin use are results of overdoses — not normal use of the drug.
Those most at risk of developing lactic acidosis while on metformin include people with conditions such as kidney or liver issues, a history of heart attacks or acute heart failure, and frequent alcohol consumers.
Hypoglycemia (low blood sugar)
Metformin only very rarely causes hypoglycemia in people with diabetes who are not taking insulin injections, because it doesn’t increase insulin production like many other diabetes medications.
Call your doctor if you have any symptoms of hypoglycemia, which can include:
- Weakness
- Tiredness
- Nausea
- Vomiting
- Stomach pain
- Dizziness
- Lightheadedness
- Abnormally fast or slow heartbeat
“Fake” low blood sugar
While this isn’t a negative side effect, it’s an important situation to be aware of when starting on metformin.
When you drop your blood sugars to a “normal” range after running consistently high for a while, you may experience “fake” symptoms of low blood sugar.
Because your body has become used to high blood sugars, returning to normal levels may make you feel dizzy, lightheaded, nauseous, and extremely hungry.
This should pass after a few days to weeks as your body adjusts to your new normal blood sugar levels.
If you’re feeling low, always check your blood sugar to make sure you aren’t experiencing a real low blood sugar (typically considered to be below 70 mg/dl) that requires treatment with glucose.
Allergic reactions to metformin
Allergic reactions to metformin are rare, but they do happen. Symptoms of an allergic reaction can include:
- Rash
- Hives
- Itching
- Swelling of the face, lips, or tongue
- Difficulty breathing
If you experience any of these symptoms, seek medical attention immediately.
How long after taking metformin do side effects start?
Metformin’s side effects usually appear quickly after the first dose, and while they improve for some people after a few weeks, others may find they persist.
Others may not even begin to experience side effects until they’ve been taking the drug for months or a year.
Reducing metformin side effects
Fortunately, there are a few things you can do to alleviate, lessen, or altogether prevent the side effects caused by metformin.
Take your dose during your meal, not before
One key step is to take your metformin dose halfway through eating your meal, rather than before you start eating.
People taking the medicine report far less stomach distress if there is already food in their stomach by the time their metformin dose is being digested.
Start with a very small dose
By starting with an extremely small dose, instead of the full dose your doctor would normally prescribe for your height and weight, you may be able to drastically reduce those initially uncomfortable side effects.
If this is an approach you’d like to try, talk to your doctor about adjusting your doses in order to give your body time to acclimate to the drug.
Read more in: Metformin Dosage Guide (Min and Max Doses).
Ask for the “extended-release” version
Too often, doctors prescribe the regular (standard- or immediate-release) version without any consideration for the extended-release (ER or XR) version.
When using the ER/XR version of metformin, the medication is gradually released over several hours instead of immediately. This process effectively lessens the unwanted gastrointestinal symptoms.
Although it may be more expensive, this approach can significantly decrease the overall side effects of metformin.
However, you may need to try the regular version first. This is so your doctor can tell your insurance company that you tried it and it wasn’t the right medication for you. Then they will be more likely to cover the extended-release version.
Take your dose whole
It is important not to crush, break, or chew your metformin tablets, as doing so could cause them to be absorbed more quickly and increase the risk of side effects.
Combine it with other diabetes medications
If your healthcare provider intends to start you on a GLP-1 drug, the side effects of metformin can actually help counterbalance the constipating side effects of many of the medicines in this class.
GLP-1 medications for diabetes currently available in the United States include the following:
- Dulaglutide (brand name Trulicity)
- Exenatide (Byetta)
- Exenatide extended release (Bydureon bcise)
- Liraglutide (Victoza, Saxenda)
- Lixisenatide (Adlyxin)
- Semaglutide (Ozempic, Rybelsus)
By including both types of drugs in your diabetes management plan, the side effects may help essentially balance each other out.
Read more in: Metformin Combination Drugs for Type 2 Diabetes.
Try something else
There are some people who simply do not tolerate metformin. If you find you cannot bear the side effects, talk to your healthcare team.
If you choose to stop taking metformin, they can help you find a different type of diabetes medication to manage your blood sugar levels.
Read more in: The Best Alternatives to Metformin for Type 2 Diabetes Management.
Who should avoid taking metformin?
As with all medications, there are certain people who shouldn’t take metformin.
The primary concern with metformin centers on its potential to increase the risk of excessive lactic acid production. Effectively removing lactic acid from the body requires proper functioning of the organs.
If you have been diagnosed with kidney or liver issues
Your liver and your kidneys both play a critical role in clearing metformin from your body. If you have severe kidney or liver disease, you should not take metformin.
That said, recent guidelines have become more permissive relative to kidney disease, recognizing that metformin can be safe for use in people with mild to moderate chronic kidney disease (CKD) under certain conditions.
If you have mild to moderate CKD, speak with your doctor about whether metformin use is safe in your situation.
If you have had a heart attack or acute heart failure
Serious heart conditions can affect how much blood is pumped to your kidneys, which can reduce your overall kidney function.
Because the kidneys play a crucial role in clearing metformin from your body, people with a history of heart issues should not take metformin.
If you drink alcohol often or in large quantities
Given that metformin’s effectiveness depends on healthy liver and kidney function, regular heavy alcohol consumption can heighten the risk of complications when taking metformin.
Read more: Metformin & Alcohol: Can You Drink While Taking Metformin?
If you will be having surgery or an imaging test
Lack of fluid and food prior to surgery can increase the burden on the kidneys and raise the risk of high blood pressure. The contrast dyes used in certain imaging tests, such as MRIs, can interact with metformin and cause a drop in kidney function.
Speak with your doctor about how to handle your metformin regimen if you will be having one of these procedures.
Debbie Carroll
I have been on Metformin ER since March and I am having muscle pains in my arms and shoulders that is steadily getting worse!
Christel Oerum
That does not sound sustainable, please consult with your doctor as soon as possible. If your joints feel stiff it could be a frozen shoulder (https://diabetesstrong.com/frozen-shoulder-diabetes/) or something else altogether. However, your doctor should be able to guide you
Rafael
I am feeling the same sympton and I do not know what can I do. Please, would you like tell me something.
Mac
I was diagnosed 7 days ago and put on Metformin 500mg. My doctor explained the diarrhea issue, but didn’t warn me of the stress associated with it. I have uncontrollable episodes 4-5 hours after each dose. I am a professional with business meetings. I can’t concentrate with the fear of a unwanted episode. How long before I can request the ER version?
Christel Oerum
I would suggest reaching out to your doctor ASAP and request the ER version. No need to suffer
Mary Layzell
Can you help me?
My husband has been on Metformin for Type2 Diabetes for almost 2 years
He has become very bad- tempered and uncooperative so much so I feel I can no longer live with him . Could this be due to this drug or a low blood sugar which usually between 5 and 6 before and after meals.
We are both76 yrs. old
Ginger Vieira
Mary!
I’m so sorry to hear what you’ve both been going through. Can low blood sugars make a person angry? Absolutely. However, technically metformin cannot cause low blood sugars. I would guess that something else is going on and I hope you can take action sooner than later in seeking professional help for both him and yourself. He may need a deeper evaluation from a doctor? Big changes in a person’s personality like that shouldn’t be taken lightly and could signify a slew of other potential health issues.
Don’t forget to take care of YOURSELF in the meantime — maybe talking to a therapist could be helpful? And make sure you are safe. Reach out to friends — don’t go through it alone.
Wishing you the best,
Ginger
Phyllis H
Since diabetics often suffer pain, it might be helpful to look into any pain medications your husband takes. Since the opioid crisis many doctors now prescribe Tramidol for pain. My husband and I both found ourselves to be short tempered and irritable. Ask you doctor.
Barry Allen
in response to laura beltran 4/30 of being tired that was what i was, I researched it and added blood lab for b-12, it was single digits. i now take b-12 oral prn and make sure that it is tested every 6 months. i have been on metformin for 15 years i am approaching 80 yo.
Becca
Do you have any suggestions for someone that has continued diarrhea after 3 years of taking the Metformin XR? It’s quite annoying because when I have to go I have to go ASAP and I have had some close calls. Sorry to be so abrupt but I’m getting very tired of this side effect as it makes it difficult to plan outings. I don’t really have a time after taking my dosage that it will happen so I could plan around that amount of time. It’s summer time and hiking season and I’m afraid to participate. I am in a research study and won’t be able to get a different drug for another 2 years as Metformin is part of the study. The only thing that helped was taking an iron supplement while I had an issue with low iron stores. That issue is resolved now and I no longer need to take iron. Now my body has returned to the same.
Ginger Vieira
Hi Becca!
If you’re still having severe digestion issues on metformin after 3 years, I would think you could be removed from the study due to an “adverse reaction” to the drug. You shouldn’t have to suffer!
I also just want to pose the question: are you sure your stomach issues are from metformin? Just in case it might be something you’re eating? Gluten?
I’m sorry I can’t be more helpful!
Janie Montez Mendoza
I had been taking metformin for 2 years on my 2nd year I developed severe diarrhea and every primary physician just gave me diarrhea meds or over the counter meds I was truly dieing by the 10th month of this condition until I noticed my stools had small white specks and later I started seeing stringy things in my stools so I researched for about a week on the internet and found out I was infested with pinworms that was so disgusting I soon realized that my diarrhea always happened right after I took the medication, so I stopped taking it and the over the counter meds didn’t work so I looked up home remedies online and it worked the pinworms came out all at one in a braided stool. So I peeped one sample and put rubbing alcohol and put in a clean compact inside a ziploc bag so the doctors didn’t want to see the specimen or photos I took during the time all this was happening to me. I was very upset I had to cure myself and spend some much money on incompetent doctors that won’t don’t care about their patients. So I had lost all my good bacteria from the diarrhea and the bad bacteria took over my intestines now that I’m cured of that I had damage to my large intestine. Colonoscopy fountain a polyp and had a biopsy thankfully it was negative but I need surgery for my prolapsed colon and I still have to wear disposable undergarments because my muscles in my intestines were left so week I have accidents and a lot of mucus drainage and now it’s clear watery drainage. Sorry to be direct but everyone should know what to look for and the out come from taking metformin when this is never disclosed to the patient taking this life threatening drug so beware!!!!!!!
Eww
Gee thanks for sharing. TMI not even remotely related to the discussion or the medication. Have some discretion, metformin sure as heck didnt infest you with worms!
Crystel
Sorry for the aweful worms and what not that’s happened to you but as someone who has graduated with a major in science. I can assure you, that metformin did not give you worms and cause you to loose muscle strength in your rectum. The intestines dont really support the muscles as your rectum is composed of more muscles, which isn’t a side effect. The only side affect that is common is severe diarrhea by metformin and as mentioned above they made an extended release version for those who suffer with the side effects and it says it’s best to take the medication while your eating like halfway through your meals and not before or after. I’m sorry a doctor is hard to find in your area. Please, consult with someone somehow to get insurance if you dont have it or ask your insurance for health providers in your area and explain to them everything or simply show them the message on here.
Jared
I was recently diagnosed as having Type 2 diabetes. I am taking Metformin. Recently I’ve discovered I cannot eat much dense protein (chicken, beef, etc.), else it gives me sulphur burps and diarrhea. I’m thinking it’s something to do with the Metformin. I think I read somewhere that Metformin slows down the digestive process, to help reduce blood sugar spikes. I’m thinking that’s causing my body to be unable to handle a lot of dense protein, because it can’t process it quickly anymore. Am I right? If so, how can I get around this limitation? Soy burgers from now on? 😀
Ginger Vieira
Jared!
Metformin absolutely wreaks havoc on the digestive system. You’ll see in the article a section about the “Extended Release” version. Your insurance company will require that you start on the normal version, but if you report to your doctor how much digestive trouble you’re having with metformin, they can usually get you insurance coverage for the Extended Release (ER) version which is much gentler on the stomach!
If I were you, that’s what I’d try first. (That being said, I LOVE a great veggie burger!!!!)
Linda
Hello
Started Metformin 10 days ago and have increased my dose to twice daily with my insulin pump working along side it , I seem to swing from diarrhoea to constipation , it’s driving me nuts as I can’t plan my day as I don’t which I’m gonna get on which day . Any suggestions would be greatly appreciated .
Ginger Vieira
Hi Linda!
I’m not sure why your body is juggling back and forth between diarrhea and constipation but many folks who struggle with diarrhea on Metformin do better when they switch to the “Extended Release” version of the drug — there’s more detail about this in the article. I would talk to your doctor ASAP and request a prescription for the “ER” version!
Could it be something else….
Hello, well if it is the metformin I’d consult your doctor ASAP, and tell them all the trouble your having, because it could be the metformin or something else never know until you go through trial and error with your physician. But for me personally I am 21 years of age and have never taken metformin not need to right now but I too suffer from bowl movements that are very different and painful. A few weeks diahherea a few normal then some constipation but mostly soft stools and I told a doctor at urgent care and she didnt diagnosed me but she did say it sounds like it could be ibs, so you never know really. All I know is since being a teen I’ve had very painful stomach issues and no doctor until her had told me it could be ibs which I had a feeling it was from my nerves and anxiety. So, I hope you find a solution to your problem soon.
Laura Beltran
I need your help, I have tried all types of medformin and I get very nauseated. At this moment I feel so weak and have not been able to get out of bed. If someone can give me some advise PLEASE.
Started with 500, then 1000 and now 2000 and it always the same.
Is there any other medication out there?
Christel Oerum
Sorry to hear that your medication isn’t working for you. Yes there are other medications available, but please don’t discontinue or switch medication without consulting your doctor or another qualified trained medical professional
Cindy
After taking Metformin in the morning I find myself rushing to the bathroom within three hours, but not everyday. My everyday breakfast is always cereal with extra walnuts and was wondering if adding more protein to my breakfast would help with diarrhea?
Ginger Vieira
Hi Cindy!
You’ll find in the article that this is VERY common — and there is something you can do about it with your doctor’s help. Report your severe diarrhea to your doctor and ask for the “ER” extended release version of the drug. Your insurance company will require you to prove that you tried to normal version first, but with your doctor’s note that you responded poorly to it, you should be able to get coverage for the “ER” version. Many patients report that the “ER” version resolves the issue of diarrhea. Good luck!
Cindy
Thank you Ginger. I was able to take the ER version and did well with it for a while, but found it difficult to swallow due to the larger size, therefore my doctor prescribed the regular and smaller metformin medicine which is causing the problem.
Jill
I am type 2 and was prescribed the ER version of metformin and it’s been about 3 mos now. The constant diarrhea has not subsided and my Dr said I could take imodium. It doesn’t help a whole lot and my goal is to decrease the amount of meds I take not increase! Anyway I’m giving in three more months and it that time I am going to be faithful in my low carb intake. I’m hoping my A1c goes down enough to at least lower the dose 🙁
Tara
I just wanted to clarify the part of the article that references Metformin Extended-release. The article states that providers “prescribe the regular version without any consideration of the extended-release (ER) version.” As an RN who was recently diagnosed with DM2, I can say that for most insurances to pay for the Metformin ER version, the Metformin orginal must be tried first. This is the same for Rheumatoid medications as well. The cheaper version must be tried first before the more expensive versions are prescribed. Thanks.
Ginger Vieira
Hi Tara!
Thank you for explaining that crucial detail — I will add it to the article.
Ginger