Disclaimer: None of the information in this article constitutes medical advice, and is just the opinion of the writer(s). We recommend that patients follow their doctor’s guidance in regard to diabetes treatment.
Metformin is a prescription medication which is approved by the Food and Drug Administration (FDA) to treat type 2 diabetes. It’s one of the most commonly-prescribed diabetes medications.
Metformin is the shortened version of the drug name, and the drug is alternatively referred to as metformin hydrochloride or metformin HCL.
But is metformin shown in research studies to reduce blood sugar levels? And if so, by how much? Does this medication also cause weight loss? And how do real metformin users rate and describe the effects of the drug?
In this article we'll answer all of these questions and more, as we review clinical studies on metformin to determine if the drug reduces blood sugar, and if it causes weight loss.
We'll discuss side effects, review clinical studies on metformin in pre-diabetics, and explain why some batches of the drug were recalled in 2022.
We'll also feature patient reviews of metformin, explain how the drug works, and explain if certain foods or alcohol need to be avoided while on metformin.
Does Metformin Reduce Blood Sugar?
Metformin has been extensively studied for its effects on blood sugar.
A medical review published in the Diabetes, Obesity & Metabolism journal referred to metformin as the “gold standard” for type 2 diabetes treatment.
The study authors found that metformin typically results in a decrease in glucose levels of around 10-30% over time.
A 1997 clinical trial compared the efficacy of metformin for type 2 diabetes at various doses: 500 milligrams (mg), 1000 mg, 1500 mg, 2000 mg, and 2500 mg.
Metformin reduced HbA1c levels (a marker for blood sugar) by 0.9% in the lowest-dose group and 2% in the highest-dose group compared to a control group taking placebo pills. The trial participants taking metformin experienced continual blood sugar decreases, while the trial participants taking placebo pills experienced continual blood sugar increases.
A 2017 medical review concluded that metformin is still one of the most effective drugs for treating and managing type 2 diabetes, but the study authors noted that metformin doesn’t have as much cardioprotective effect as some newer diabetes medications.
As we highlighted in our recent Victoza reviews article on a newer-generation diabetes drug, Victoza not only reduces blood sugar but also reduces blood pressure in hypertensive patients, suggesting cardioprotective effects.
We will conclude from the available research that metformin is effective for treating type 2 diabetes, which is unsurprising given that the drug is approved by the FDA for that outcome.
Can Metformin Prevent Diabetes?
Because metformin is one of the most common treatments for type 2 diabetes, patients are often curious about whether the drug is also effective for prediabetes, which is defined as blood sugar levels above the healthy range but not yet in the diabetic range. Diabetes is typically defined as blood sugar levels at or above 126 mg per deciliter (dL).
It’s worth noting that the American Diabetes Association recommends metformin for the treatment of prediabetes, but the drug is not currently approved by the FDA to treat prediabetes.
A meta-study analyzed data from three clinical trials on metformin for prediabetes.
The researchers found that the drug was effective at both an 850 mg twice-daily dose and a 250 mg twice-daily or three times daily dose. For every 10 prediabetic patients treated with metformin, around 1 of them would be prevented from getting diabetes according to the above-linked study.
We hope that more clinical trials are funded evaluating the safety and efficacy of metformin for prediabetes. If more trials can replicate these results, the FDA may consider approving the drug for the treatment of prediabetes.
Does Metformin Cause Side Effects?
Metformin does cause side effects in some individuals.
Medical research shows the majority of metformin side effects are gut-related: 20% to 30% of patients on metformin experience gastrointestinal side effects such as nausea and vomiting.
Metformin has a “black box” warning on its FDA label, which is the most severe type of warning for a prescription medication. It indicates a side effect that may be life-threatening.
In the case of metformin, this black box warning describes a rare but severe side effect called lactic acidosis, and according to the medical review linked above, this side effect occurs in around 1 in 30,000 metformin patients. This side effect primarily occurs in diabetic patients with liver and kidney dysfunction.
Even though the risk of lactic acidosis is rare, it seems prudent for diabetic patients with liver and kidney dysfunction to speak with their doctor about potential alternative diabetes medications which don’t confer this risk.
Does Metformin Cause Weight Loss?
Many patients are curious about whether metformin causes weight loss, and whether it’s safe to take for weight loss alone.
As we detailed in our metformin weight loss article, the medication does cause weight loss in overweight and obese patients at a dosage between 1000 mg/day and 2,550 mg/day, at least according to the clinical trials that we came across.
This makes metformin a good option for overweight or obese diabetic patients to speak with their doctor about, because the drug can potentially benefit two separate health conditions.
Metformin is not currently FDA-approved for weight loss alone. It also doesn't appear to have been thoroughly studied in non-diabetic patients, which makes it challenging to determine whether or not the drug would be effective in non-diabetic overweight or obese patients.
Real People Review Metformin
A YouTube creator named Erica Rodriguez shared her experience after using metformin for a month:
A YouTube creator named "The Hangry Woman" discusses how she avoided metformin side effects:
Why Was Metformin Recalled?
In January of 2022, the FDA announced a US recall of metformin manufactured by a company called Viona Pharmaceuticals Inc.
The metformin extended-release tablets manufactured by Viona Pharmaceuticals had levels of a compound called N-Nitrosodimethylamine (NDMA) which exceeded federal limits.
According to the Environmental Protection Agency (EPA), this compound is both toxic and carcinogenic (potentially cancer-causing).
Since metformin is available from a variety of manufacturers, it may be sensible to avoid purchasing metformin from this company in the future, as this recall indicates a potential lack of appropriate quality control measures.
We haven't come across any information suggesting that patients were harmed from this recall, or any information about potential class-action lawsuits.
Patients Review Metformin
Metformin has been reviewed over 600 times on Drugs.com, which is a website that allows users of prescription medication to publish reviews of their experience on the drug.
We cannot verify the accuracy or authenticity of any reviews on this site.
At the time of updating this article, metformin has an average review rating of 5.9 out of 10 for type 2 diabetes.
The top review of metformin for type 2 diabetes is written by a user named “Averageguy50yrsold” who claims that he lost weight and improved his quality of life:
“So far Metformin has been a wonder drug for me. I'm dropping weight, not hungry at all, and my energy levels are up. I'm taking one with each meal. I'm losing 5+ lbs a week and it's all from my stomach fat.”
The top negative review comes from a user named “Ana ruby” who gave the drug a 1/10 rating and claims it caused uncomfortable side effects:
“I was taken metformin for 5 months now and am losing a bit of my weight but one thing my concerned is I'm having diarrhoea all time and the worst part is if I'm going out like for walking or groceries middle of this situations I really need to go toilet straight away otherwise I will be poop on my jeans. This is really disturbing for me”
Is Avoiding Certain Foods or Alcohol Required?
Metformin’s FDA label does not currently indicate any specific foods to avoid while using the drug.
That being said, we know that high-glycemic foods like baked goods and white bread cause blood sugar spikes, so it may be logical to avoid such foods while using the drug.
Eating a whole foods diet free of any packaged or processed foods could benefit diabetic patients but is not required, and it makes sense to speak with a doctor prior to making any significant dietary changes.
A 2016 medical review found that diets rich in whole grains, fruits, vegetables, nuts and beans, with moderate-to-low alcohol consumption can improve blood sugar control.
There is a section on metformin’s FDA label indicating that excessive alcohol use may make the drug more dangerous. The label references excessive alcohol use “acute or chronic,” which suggests that even one night of binge drinking may significantly impact risk.
Given this risk, it's important for patients with alcohol use issues to speak honestly with their doctor before taking metformin, because if they're unable to abstain from alcohol, their doctor may recommend a different diabetes medication that doesn't have a potential alcohol interaction.
Are There Signs Metformin is Working?
Patients are often curious about whether there are early signs that a drug is working. In the case of metformin, we haven't come across any clinical evidence that physical symptoms can be signs that the drug is working or not working.
Metformin is a diabetes drug, so the only way to test whether or not the drug is working is to regularly get blood glucose levels checked.
Doctors will often require type 2 diabetics to get an at-home blood sugar monitor and test their glucose levels once or even multiple times per day.
There are no well-established physical symptoms of blood sugar normalization based on medical literature that we can identify. Patients could theoretically experience fewer symptoms of hyperglycemia while on metformin, such as blurred vision and frequent urination, but these symptoms will vary patient-to-patient.
Metformin vs. Berberine
Berberine is a dietary supplement that has been studied for its effects on blood sugar, so patients are often curious about how its effectiveness compares to that of metformin.
A clinical trial published in the Metabolism journal compared the effectiveness of berberine and metformin in diabetic patients.
Berberine was as effective as metformin for reducing blood sugar levels, but also had favorable effects on cholesterol levels (like reducing triglycerides), while metformin did not.
These early results don’t necessarily prove that berberine is more effective than metformin; much more research would be needed replicating these results to prove that.
We do not recommend that patients discontinue metformin in favor of berberine without a doctor's approval.
These results do suggest that prediabetic patients may benefit from speaking with their doctor about berberine, given that there are no approved medications for prediabetes at the time of updating this article, and berberine appears to have a potent anti-hyperglycemic effect.
Our Blood Sugar Support Picks
Cinnamon extract is a dietary supplement that can help support healthy blood sugar levels.
A medical review published in the Annals of Family Medicine journal found that cinnamon consumption reduced fasting blood sugar levels, reduced total cholesterol levels and reduced triglyceride levels.
A clinical trial from 2006 found that cinnamon extract caused a 10.3% blood sugar reduction in type 2 diabetics, while placebo pills only caused a 3.4% reduction.
Illuminate Labs Ceylon Cinnamon Extract is our Ceylon cinnamon supplement that only costs $15 on a subscription basis.
Bulletproof Magnesium is our top magnesium supplement pick, because it's affordable (under $15 at a subscription rate at the time of updating this article) and has no inactive ingredients that we consider to be unhealthy.
We're not suggesting either of these supplements should be used to treat any medical condition, or that they're as effective as any FDA-approved medication; rather, we're just sharing information that individuals averse to prescription medication can speak with their doctor about.
Can Metformin Treat PCOS?
Patients diagnosed with polycystic ovary syndrome (PCOS) are often curious about whether metformin is effective for their condition, given that some medical studies have shown an association between PCOS and insulin dysfunction.
There have been clinical trials on metformin for reducing PCOS symptoms.
A 2015 medical review examined whether the combination of metformin and lifestyle modifications such as improved diet and exercise could improve PCOS.
The study authors found that metformin plus lifestyle changes was more effective for weight loss than lifestyle changes alone, and given that PCOS can be associated with weight gain, this is an indirect benefit.
Another meta-study, published in the Endocrine journal, found that metformin may be effective for a wider range of therapeutic outcomes in women with PCOS than weight loss alone. In particular, metformin was found to improve live birth rate and ovulation rate when combined with standard PCOS treatments like clomiphene citrate.
Metformin is not currently approved by the FDA for the treatment of PCOS.
Does Metformin Cause Diarrhea?
Diarrhea is obviously an uncomfortable side effect of some medications, and patients are often curious about whether metformin causes this side effect given that it can cause gastrointestinal symptoms in some patients.
Unfortunately, metformin does appear to increase the likelihood of diarrhea.
Metformin has been identified as a potential cause of late-onset diarrhea, which is defined as chronic diarrhea experienced by patients without a medical history of this condition, in a case report.
An animal study helped researchers understand how metformin may cause diarrhea. The researchers found that mice with low levels of a particular bacteria called Firmicutes in their gut were more likely to experience diarrhea, suggesting that the microbiome may predispose human patients to diarrhea while using metformin.
Metformin will not cause diarrhea in all patients, but it's worthwhile for patients with gut issues to discuss this risk with their doctor.
Are There Metformin Alternatives?
There are a variety of type 2 diabetes medications available on the market, some of which may be more effective for certain patients than metformin, given that every patient responds differently to different prescription medications.
Victoza is a type 2 diabetes medication. This drug was found to be slightly less effective (but not to a statistically significant degree) than metformin in a head-to-head clinical trial in diabetic patients.
Victoza is injectable, while metformin comes in oral tablets, so Victoza may be a better option for patients with an aversion to pills or an inability to swallow them.
Rybelsus is another diabetes drug that was found to be equivalently effective to metformin in at least one clinical trial, as we documented in our Rybelsus reviews article.
In that article we detailed how the two drugs were actually tested in combination, and found to yield superior results to either drug in isolation. This suggests that patients with difficult-to-treat diabetes may benefit from speaking with their doctor about taking both Rybelsus and metformin.
Overall, we have not come across a prescription medication for type 2 diabetes that’s more effective based on clinical trials than metformin.
Metformin is prescribed at a wide dosing range.
StatPearls, which is one of the largest free medical databases in the US, documents how most adults are prescribed a starting dose of 500 mg once or twice daily, or 850 mg once-daily. A maximum dose of 2000 mg once or twice daily can be prescribed using the extended-release (XR) version of the drug.
Typically, doctors will prescribe diabetic patients a dose on the lower end of the therapeutic range, because this may reduce the risk of side effects.
If the drug fails to successfully lower blood sugar at the lowest dose, the doctor can slowly increase the dose.
For both blood sugar and weight loss, metformin appears to be more effective at the higher doses of the therapeutic range than the lowest doses, which would be logically expected.
How Does Metformin Work?
Metformin decreases production of glucose by the body and increases insulin sensitivity. In many type 2 diabetics, insulin sensitivity and metabolism is dysregulated.
Not only does metformin decrease the body’s production of glucose, but it has beneficial downstream effects such as decreased intestinal absorption of glucose and therefore decreased blood levels of glucose.
The same mechanism of action in metformin that is beneficial for type 2 diabetics is likely what’s beneficial for weight loss in overweight and obese diabetics. Insulin dysregulation is associated with weight gain. There’s even a medical term “insulin-associated weight gain.”
By normalizing insulin function, metformin may reduce blood sugar levels and weight.