Why Are They Saying Metformin Is Bad for You? Separating Fact from Fear

October 11, 2025 •

4 min read

As the most widely prescribed oral medication for type 2 diabetes, with millions of patients using it globally, it is crucial to understand the concerns that lead people to ask: "Why are they saying metformin is bad for you?". While highly effective and generally safe, its reputation is sometimes clouded by publicized side effects, a rare but serious complication, and past recalls.

Quick Summary

Separating fact from public concern about metformin involves examining its common gastrointestinal side effects, the low risk of lactic acidosis, and addressing past NDMA contamination recalls.

Key Points

Not Inherently Bad: Metformin is a safe, effective, and low-cost first-line treatment for most people with type 2 diabetes.
Common Side Effects: Many concerns stem from common gastrointestinal issues like diarrhea, nausea, and bloating, which often improve with time or by using the extended-release formulation.
Rare Lactic Acidosis: The risk of lactic acidosis is extremely low and mainly affects patients with severe underlying conditions like kidney or liver disease, heart failure, or those who abuse alcohol.
NDMA Recalls Were Contamination: Past recalls of extended-release metformin were due to manufacturing impurities (NDMA), not the drug itself. The most common immediate-release form was not recalled, and the issue has been addressed.
Long-Term Vitamin B12 Risk: Prolonged use can lead to a vitamin B12 deficiency, requiring periodic monitoring and possible supplementation.
Benefits Outweigh Risks: For most patients, the benefits of controlled blood sugar and reduced diabetes complications far outweigh the known and manageable risks.

Understanding Metformin's Known Side Effects

Metformin is a first-line treatment for managing type 2 diabetes due to its effectiveness, low cost, and well-understood profile. However, like any medication, it has side effects, and for some, these can be bothersome enough to question the drug's safety. The most common issues are related to the gastrointestinal (GI) system.

Common Gastrointestinal Side Effects

Diarrhea: This is the most frequently reported side effect and often occurs when starting the medication. It tends to subside over time as the body adjusts, especially when starting with a low dose that is gradually increased.
Nausea and Upset Stomach: Many patients report feeling nauseous, experiencing bloating, or having a general stomachache. Taking metformin with food can help alleviate these symptoms.
Metallic Taste: Some people notice a persistent metallic taste in their mouth, which is a known, though less frequent, side effect.

Extended-Release Formulations Can Help

For those who experience significant GI distress with immediate-release metformin, a doctor may recommend switching to an extended-release (ER) version. ER tablets are designed to release the medication slowly, which can minimize stomach upset. This simple change can make the drug much more tolerable and improve a patient's quality of life without sacrificing its benefits.

NDMA Recalls: Separating Drug from Contaminant

In 2020, headlines about metformin recalls caused significant alarm. It is important to clarify the reason for these recalls to understand why it was a manufacturing issue and not an inherent flaw in the drug itself. The recalls involved certain extended-release metformin products that were found to contain levels of N-nitrosodimethylamine (NDMA) above the acceptable intake limit.

What is NDMA?

NDMA is an environmental contaminant that can also be introduced during the manufacturing process. While NDMA is a probable human carcinogen at high levels over a long period, the FDA reassured the public that short-term exposure at the levels found in the recalled products was not expected to increase cancer risk. The FDA found that immediate-release (IR) metformin tablets, the most common form, were not affected. Patients were advised not to stop taking their medication without consulting their doctor, as uncontrolled diabetes poses a far greater risk than the NDMA exposure.

The Lactic Acidosis Concern: Unpacking the Risk

One of the most serious but extremely rare risks associated with metformin is lactic acidosis, a potentially life-threatening condition caused by a buildup of lactic acid in the blood. The fear is rooted in an older biguanide drug, phenformin, which was removed from the market due to a high incidence of this side effect. However, metformin's risk is significantly lower and is primarily a concern for individuals with pre-existing health conditions.

Who is at Risk?

Risk factors for metformin-associated lactic acidosis (MALA) are primarily related to conditions that prevent the body from clearing metformin or lactic acid efficiently:

Severe kidney or liver impairment
Unstable congestive heart failure
Severe infection or shock
Heavy alcohol consumption

Lactic Acidosis is Extremely Rare with Appropriate Use

Multiple large-scale reviews and studies have found no increased risk of lactic acidosis when metformin is used correctly and prescribed to patients without contraindications. This is why prescribing guidelines emphasize careful patient selection and monitoring of kidney function. For the vast majority of patients without these risk factors, the risk is negligible.

The Long-Term Issue of Vitamin B12

Another long-term concern with metformin use is the potential for vitamin B12 deficiency. Long-term use of metformin, especially at higher doses, can interfere with the body's absorption of vitamin B12 in the small intestine.

Symptoms and Monitoring

Symptoms of a B12 deficiency can include neuropathy (numbness or tingling in hands and feet), anemia, fatigue, and memory problems. Due to this risk, medical guidelines recommend that patients on long-term metformin therapy, particularly those with risk factors, have their vitamin B12 levels checked periodically. If a deficiency is detected, it can be corrected with B12 supplements or injections while continuing metformin.

Evaluating the Benefits vs. Risks

When considering why people question metformin's safety, it's important to weigh the specific risks against its significant benefits. For many, the advantages of controlled blood sugar and reduced diabetes complications far outweigh the manageable risks.


Feature	Benefits of Metformin	Risks of Metformin
Effectiveness	First-line, highly effective glucose control	Mild for most, but potential for serious issues in at-risk groups
Cardiovascular Health	Reduced risk of cardiovascular events, including heart attack and stroke	None associated directly with the drug itself, but underlying conditions increase lactic acidosis risk
Weight Management	Weight-neutral, and can cause modest weight loss in some individuals	Weight gain is not a typical side effect
Safety Profile	Well-established and generally safe for most patients, especially the IR form	Gastrointestinal upset is common; rare risk of lactic acidosis and B12 deficiency
Cost and Accessibility	Inexpensive and widely available generic medication	Requires proper medical supervision, especially for dosage and monitoring

Making an Informed Decision

The perception that "metformin is bad for you" stems from a combination of common, manageable side effects, public anxiety over past recalls, and a serious but exceptionally rare complication that is largely avoidable with careful prescribing. The key takeaway is that for the vast majority of patients with type 2 diabetes, metformin remains a safe, effective, and beneficial treatment. The NDMA recalls were a manufacturing issue that has been addressed, and the risk of lactic acidosis is primarily confined to individuals with significant pre-existing conditions.

Patients should maintain open communication with their healthcare provider. They can address any side effects, discuss monitoring for vitamin B12 levels, and ensure that their overall health profile aligns with safe metformin use. A collaborative approach empowers patients to make informed decisions about their health, recognizing that for millions, metformin is an essential and life-improving medication.

For more information on the FDA's updates regarding NDMA in metformin products, visit the FDA website.

Frequently Asked Questions

No, metformin itself is not known to cause cancer. The concern arose from past recalls of certain extended-release products due to NDMA contamination, which is a probable human carcinogen. The FDA stated that short-term exposure to these levels was not expected to increase cancer risk. In fact, some studies suggest metformin might have anti-cancer effects.

Metformin does not damage healthy kidneys. However, it is primarily removed from the body by the kidneys. For individuals with existing severe kidney problems, metformin levels can build up, increasing the risk of lactic acidosis. For this reason, dosage is adjusted or discontinued based on kidney function tests.

Lactic acidosis is a rare but serious buildup of lactic acid in the blood. The risk is extremely low for people with normal kidney and liver function. It is primarily a concern for those with severe pre-existing health conditions, not an issue with metformin itself when used appropriately.

In 2020, some extended-release (ER) metformin products were recalled by the FDA because they contained levels of N-nitrosodimethylamine (NDMA) above the acceptable limit. This was a manufacturing impurity and did not affect the more commonly prescribed immediate-release (IR) metformin tablets.

The most common side effects are gastrointestinal, including diarrhea, nausea, bloating, and gas. These often occur when first starting the medication and can be managed by taking it with food, starting with a lower dose, or switching to an extended-release formula.

No, you should never stop taking metformin without consulting your healthcare provider. Uncontrolled diabetes poses a far greater health risk than the rare side effects or past recall issues. Your doctor can address concerns and, if needed, discuss alternative treatments.

Yes, long-term use of metformin can interfere with the body's absorption of vitamin B12. This can lead to a deficiency, which may cause symptoms like neuropathy, anemia, or fatigue. Regular monitoring and B12 supplementation can prevent this.

While not specifically a weight-loss drug, metformin can lead to modest weight loss and is considered weight-neutral. It does not cause weight gain, unlike some other diabetes medications. Some studies have explored its use for weight management.