Metformin is one of the most prescribed drugs on the planet with over 120 million people taking it for type 2 diabetes management and weight loss.
While it 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 those side effects.
How metformin works
Metformin works by reducing the amount of sugar your liver releases throughout the day, and by increasing your body’s sensitivity to insulin.
It also reduces the amount of glucose 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)
- Nausea or vomiting
- Bloating and constipation
- Overall uncomfortable stomach
- Decreased appetite and weight loss
- Muscle pain or cramping
Some people also experience an unpleasant metallic taste in the mouth after taking metformin.
Metformin’s side effects usually appear quickly after your first dose and while they can improve for some after a few weeks, others may find they persist.
That being said, some people may not experience uncomfortable side effects until they’ve been taking the drug for months or a year.
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. Left untreated, B12 deficiency can cause significant nerve damage, leading to a diagnosis of peripheral neuropathy.
If a doctor isn’t aware of this risk, they could misdiagnose the cause of a patient’s neuropathy as the result of high blood sugar levels when it may be the side-effect of 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 B12 levels tested annually. Anyone with inadequate levels should start taking a B12 supplement immediately.
There have been a significant number of studies on metformin’s risk of inducing lactic acidosis — a state in which lactic acid builds up in the body, which can be fatal.
Metformin comes with a “black box” warning about this risk, the most severe warning that the Food and Drug Administration (FDA) issues.
However, the greater majority of studies concluded without any cases of lactic acidosis according to “The Phantom of Lactic Acidosis due to Metformin in Patients With Diabetes” in the American Diabetes Association’s (ADA) Diabetes Care journal.
“The number of documented cases of metformin-associated lactic acidosis is small when one considers how widely metformin is used,” explains the ADA. “That metformin has been used safely in patients with contraindications can be viewed as evidence that it does not cause lactic acidosis.”
Almost all cases of lactic acidosis from metformin use are results of overdoses – not normal use of the drug.
“Cases of lactic acidosis from metformin overdoses, particularly in young people without risk factors, suggest that metformin can cause lactic acidosis primarily if given in large doses.”
Patients at greatest risk for developing lactic acidosis while taking metformin are those with other conditions including kidney or liver issues, a history of heart attacks or acute heart failure, and patients who drink alcohol frequently.
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:
- Stomach pain
- Abnormally fast or slow heartbeat
“Fake” low blood sugar
While this isn’t a negative side effect, it’s important 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 (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:
- Swelling of the face, lips, or tongue
- Difficulty breathing
If you experience any of these symptoms, seek medical attention immediately.
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
The first is to take your metformin dose halfway through eating your meal rather than before you start eating.
Patients report far less stomach distress if there is already food within their stomach by the time their metformin dose is being digested, too.
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.
Talk to your doctor about adjusting your doses in order to give your body time to acclimate to the drug.
Read more: Metformin Dosage Guide (Min and Max Doses)
Ask for the “extended release” version
Too often, doctors prescribe the regular version without any consideration for the extended-release (ER) version.
When taking the ER version, each dose is slowly released over the course of several hours versus all at once, which significantly reduces the unwanted gastric side effects. While it does cost more, it could greatly reduce metformin’s side effects.
However, you may need to try the regular version first. This is so your doctor can tell your insurance company you tried it, and it wasn’t the right medication for you. Then they will be more likely to cover the more extended “ER” version.
Combine it with other diabetes medications
If you and your healthcare team intend to start you on a GLP-1 drug like Victoza or Byetta, the side effects of metformin can actually give balance to the constipation side effects of most GLP-1 drugs.
By taking both drugs as part of your diabetes management plan, the side effects of both drugs essentially balance each other out.
Read more: Metformin Combination Drugs for Type 2 Diabetes
Try something else
There are simply some people who do not tolerate metformin. If you find you cannot bear to endure the side effects, talk to your healthcare team.
If you choose to stop taking metformin, your healthcare team can help you find a different type of diabetes medication to improve your blood sugar levels.
Read more: The Best Alternatives to Metformin for Type 2 Diabetes Management
Who should avoid taking metformin?
Like most medications, there are a few types of patients who shouldn’t take metformin.
For metformin, the concern is based largely on the fact that it can raise your risk of producing too much lactic acid. Clearing that lactic acid from your body requires healthy organ function!
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 your kidneys or liver are already struggling to function, you wouldn’t want to add to their workload by taking metformin.
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, patients with a history of heart issues should not take metformin.
If you drink alcohol often or in great quantity: Again, because metformin relies on the function of a healthy liver and kidneys, a person who drinks large amounts of alcohol regularly would be putting themselves at an increased risk for other issues if they took metformin.
Read more: Metformin & Alcohol: Can You Drink While Taking Metformin?
While metformin is generally well-tolerated, it can have side effects like any medication.
The most common side effects are mild and gastrointestinal in nature, but more serious side effects like lactic acidosis and vitamin B12 deficiency can occur.
If you experience any symptoms while taking metformin, talk to your doctor about adjusting your dose, switching to extended-release tablets, or trying a different medication.
If you experience any serious or life-threatening symptoms, seek medical attention immediately.
Giles Foxx Jr says
Im 78 year old male just started taking metformin. I have an enlarged prostate and have noticed having more trouble urinating than before. Do you know if metformin causes and trouble with BPH ?
Christel Oerum says
I’m not sure so can’t be of much help. I’d suggest calling your doctor and letting him/her know, you might need a different dose or different medication
Stefan Gronewold says
Hello to all!
I have Diabetes Type 2 and live in Germany. I had in March 2017 a Heart Attack and two Stent’s. Since September 2020 i used Metformin. I become it from my Diabetologist. I have no side Effects from Metformin, and no Kidney Problems, so i think i can used Metformin further! People with little Money or Older People or Pensioners must not pay Metformin in Germany, they became it from the Health insurance.
I am curious If anyone else has experienced enhanced taste when taking metformin. Shortly after my second dose everything was way more sweet/salty than normal.
Christel Oerum says
That’s a fascinating question. I don’t know the answer to that, but maybe someone else can weigh in
Loretta Barfield says
My taste is also distorted. In addition to what you are experiencing, nothing tastes as I was used to it tasting.
Carroll Tandetzke says
This is a very interesting article. Please, share more like this!
Chadwick foster says
I am always feeling sick whenever I eat something heart rate fastens and have thinking issues and upset stomach and need help 800 mg metformin prescribed and I just feel sick all the time should I quit taking it and just eat meat.?
Christel Oerum says
Please don’t stop taking your meds without consulting your doctor. Instead, contact your doctor and make him/her aware of your issues. An alternative might be a slower release tablet or a different type of medication
I was wondering if Metformin takes time to rebuild effectiveness after a period of eating higher-carb meals? For example, I’ve been on Metformin for a year and I have eaten consistently well during that time, but Christmas and New Year consisted of some pretty poor eating on my part but I was taking Metformin throughout. I’m now back to regularly scheduled eating but have found that I’m not reaping the typical benefits of proper eating while on Metformin. Does it take some time for the effectiveness of Metformin to rebuild after a period of poor eating?
Ginger Vieira says
It’s really not an issue of your metformin not working as much as it’s about your diet changing. Your metformin was dosed properly for your non-Holiday normal diet. And then you ate more calories and carbs for a couple of weeks thanks to the holiday season (and the dark cold winter, if you live where I do!)…and so your metformin dose simply wasn’t high enough to keep your blood sugars in range with that increase in your diet.
Now that you’re back to your normal routine, I suspect you’ll find your blood sugars will go back to your normal range within a week or two of eating more balanced.
Let me know if this is helpful! If your BGs continue to run high, that’s worth making a call to your doctor to discuss adjusting your medication doses.
I too have suffered from diarrhea for at least four months after having my dose of metformin doubled. As of yesterday I was taken off of it for the next three months. A colonoscopy has been scheduled just to make sure that it’s nothing else. It’s destroying my life. No energy etc.
One suggestion I found to curb this diarrhea is I now take metformin-ER. I take my recommended doses(500mg each tablet) but I break pills in half. Example my dose is 2 daily, I take half tab in morning other half at around 5 pm.,next half before bed 10 or so, then last one at around 5am when I’m always up to head to bathroom.
Until I did that I could barely leave my house and the stomach upset was just terrible. I do hope my suggestion works, I did discuss this w provider first..
I have been on metformen for maybe 15 years. Iasked my doctor if it was safe and he said yes but I was reading some of the side effects and seem to have a few of those like shortness of breath, extreme exhaustion tired all the time. Im76 years old and always had lots of energy Until the last couple years. He added B- 12 shot once a month about a year ago Most days I want to just stay in bed. I know my age might have something to do with it but not to the extreme that I feel I’m also lightheaded and have occasional dizzy Spells I just want to feel better
Christel Oerum says
Not feeling well is so frustrating. My suggestion would be to take these concerns to your doctor and have a good discussion. It might not be the Metformin but something completely different, regardless it should be looked into. If you find that your doctor doesn’t listen (unfortunately that happens sometimes) I’d recommend you look for another doctor
My 14 year old daughter was recently prescribed Metformin. She has been having terrible stomach pains. She takes the medication right after eating supper each night, but is really struggling. Are there any foods that could help with the stomach pains? I’m having her try eating yogurt with good probiotics and wonder if Sweet Acidophilus milk might help.
Ginger Vieira says
Food isn’t going to help this. If you read this article thoroughly, you’ll see that upset stomach is a common side-effect, and the best solution is to ask her doctor for the “extended release” version. Your insurance will require that she tried the cheaper basic version first, but a doctor can document that she didn’t tolerate well and that she needs the “extended release” version instead.
Contact your daughter’s healthcare team ASAP!
georgina Lewis says
I was told by my doctor to take Metformin half way through your meals
BE Dempsey says
Yes i was taking mine either a few minutes before or after meals and occasionally running to the toilet, then i started taking them with/during meals and no problems.
I’m 54 yr old, 5’4” and 236 lbs… I’m healthy but know that I really need to drop to at least 200lbs. I eat low carb and swim 1 hour 5 days a week and can’t loose not 1 lb! I”m thinking that Metformin could help can I take small dose before bed, I’m ok waking up to do my business then go back to sleep. What dose is good to start with?
Christel Oerum says
That’s a discussion you’d have to have with your doctor. He/she should go over it with you when the prescription is written