In short
- MedlinePlus says metformin may rarely cause a serious, life-threatening condition called lactic acidosis.
- The warning names kidney disease, some other conditions, some other medicines, heavy drinking, and sudden illness or dehydration as reasons for extra care.
- Kidney checks before and during treatment, and planning around surgery or scans with injected dye, both come from this warning.
- In 2020, FDA recommended recalls of some extended-release metformin because of an impurity, and told patients not to stop their medicine without talking to a clinician first.
Lactic acidosis
MedlinePlus opens its metformin page with an important warning: the medicine may rarely cause lactic acidosis, which it describes as serious and life-threatening. As the name suggests, the condition involves lactic acid in the blood. The MedlinePlus metformin page does not go into the chemistry, and this page does not try to.
Two words in that warning matter equally. "Rarely" means it is uncommon. "Life-threatening" means it is worth knowing the symptoms anyway, and knowing what to do. The symptoms MedlinePlus lists, and the point at which to call a doctor or emergency services, are set out in the side effects guide.
Who the warning names
MedlinePlus asks people to tell their doctor about the following before taking metformin, because of the lactic acidosis warning. This is a list of things to raise, not a test to apply to yourself.
- Kidney disease. MedlinePlus says that if you have kidney disease, your doctor will probably advise against metformin.
- Age over 65.
- A past heart attack or stroke, and heart disease or liver disease.
- Diabetic ketoacidosis, which MedlinePlus describes as blood sugar high enough to cause severe symptoms and need emergency treatment, or a past coma.
- Certain other medicines. MedlinePlus names a small number of medicines that may raise the risk when taken with metformin. Rather than reproduce that list here, this site's advice is the source's own: tell your doctor and pharmacist about every medicine, vitamin, supplement and herbal product you take.
- Alcohol. MedlinePlus says to tell your doctor if you drink regularly or sometimes drink a lot in a short time, because alcohol raises the risk of lactic acidosis and can also lower blood glucose. It suggests asking your doctor how much alcohol is safe for you.
Illness, vomiting and dehydration
The same warning covers short-term illness. MedlinePlus says to tell your doctor if you have recently had, or develop during treatment, any of these:
- a serious infection;
- severe diarrhea, vomiting or fever;
- drinking much less fluid than usual, for any reason.
MedlinePlus explains that your doctor may pause metformin until you recover. That decision is the doctor's, which is why the source frames this as something to report rather than something to manage alone.
NIDDK makes a related general point for everyone with diabetes: ask your care team ahead of time whether you should adjust your meal plan, activity or medicines in special situations, such as being sick or fasting. Having that answer before you need it is far easier than working it out while unwell.
Why kidney tests come up
Kidneys come up repeatedly in conversations about metformin, for two separate reasons.
The medicine. MedlinePlus says your doctor will arrange tests before and during treatment to check how well your kidneys are working and how your body is responding to metformin, and asks you to keep all appointments with your doctor and the laboratory. Kidney disease is also the first condition named in the warning above.
The diabetes. Separately from any medicine, diabetes itself affects the kidneys. NIDDK says diabetes is the leading cause of kidney disease and that about 1 in 3 adults with diabetes has kidney disease. It usually develops slowly over many years, and NIDDK says most people with diabetic kidney disease have no symptoms, so the only way to know is to be checked.
NIDDK describes the two usual checks: a urine test for a protein called albumin, and a blood test that shows how well the kidneys are filtering blood. It says the best way to slow or prevent kidney disease from diabetes is to work toward blood glucose and blood pressure goals.
So if a clinician asks for kidney tests, it may be about the medicine, about the diabetes, or both. It is reasonable to ask which, and what the results mean for you.
Surgery and scans with injected dye
MedlinePlus says that if you are having surgery, including dental surgery, or any major medical procedure, you should tell the doctor that you take metformin.
It also singles out x-ray procedures in which a dye is injected, and says to tell your doctor about any that are planned, especially if you drink or have drunk large amounts of alcohol, or have or have had liver disease or heart failure. MedlinePlus explains that metformin may need to be paused around such a procedure, and that your doctor will tell you exactly when to pause it and when to restart.
Because more than one clinician may be involved, it helps to mention metformin to each of them: the person who prescribes it, the team arranging the scan or operation, and the dentist. MedlinePlus also suggests carrying a list of your medicines in case of emergencies and bringing it if you are admitted to hospital.
Worked example
This example is invented and describes no real person. A letter arrives for a scan that uses an injected dye. Before the appointment, the patient phones the clinic that prescribes their metformin to say a dye scan is booked, and asks the imaging team to note it too. Neither call involves deciding anything alone; both make sure the people who give the instructions know the full picture.
The 2020 extended-release recalls
In 2020 FDA published questions and answers about an impurity called NDMA (N-nitrosodimethylamine) found in some metformin. The points below are from that FDA page, which was last updated in September 2020.
- FDA recommended recalls of certain metformin products that might contain NDMA above the acceptable intake limit. It said it had recommended that five manufacturers recall lots of their extended-release metformin.
- FDA's testing found raised NDMA levels in some extended-release metformin, but not in the immediate-release form it tested.
- FDA asked makers of extended-release metformin to assess their risk and test at-risk batches before release.
- On cancer risk, FDA said it does not expect nitrosamines to cause harm at or below the acceptable intake limit, even over a lifetime, and that it did not expect shorter-term exposure above the limit to raise cancer risk. Long exposure above acceptable levels might raise it.
- Most importantly for patients, FDA said people taking recalled metformin should keep taking it until a doctor or pharmacist provides a replacement or a different option, because stopping without first talking to a health care professional could be dangerous for people with type 2 diabetes.
FDA's page directs readers to its drug recalls listings to check whether a particular product has been recalled. Recall information changes, so the current FDA listing, and the pharmacist who supplies your medicine, are the places to check, not this page.
Keeping risk in perspective
FDA's general guide on medicines describes every medicine as having both benefits and risks, and says the right balance depends on the person's situation, with expert advice from their doctor or pharmacist. The risks on this page are real, and the sources take them seriously. They are also described as rare, and the safeguards the sources describe (kidney checks, telling clinicians about illness, procedures and other medicines) are routine parts of care.
If something on this page worries you, the useful next step is to write the question down and bring it to the clinician who prescribes your medicine. The questions guide collects several you might use.