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How long do you have to stop metformin before a CT scan?

Metformin should be stopped at least 48 hours prior to a CT scan
Rebecca Rovenstine writer headshot
By | Jan. 15, 2025 Medically reviewed by Ashley Wong, Pharm. D.
Rx bottle: How long do you have to stop metformin before a CT scan?
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Metformin, one of the most popular drugs to treat diabetes, comes with a rare side effect of lactic acidosis. Lactic acidosis can even be fatal, especially for those with impaired kidney function. Kidneys can be further taxed from the contrast during a computed tomography scan (CT scan), meaning it’s vital for those taking metformin to discontinue use before the diagnostic imaging appointment.

Below, we’ve spoken with the medical experts to learn everything you need to know about the procedures metformin users should take before a CT scan.

Why stopping metformin is crucial before a CT scan

“If you are scheduled for a CT scan that uses contrast dye, your doctor may advise you to stop taking metformin for a short period before the procedure,” said Dr. Andrew Youssouf, a dual board-certified physician and the Medical Director at Ikon Recovery Centers in New Jersey. 

Metformin, sold under the brand names Glucophage, Fortamet, and Riomet, is an anti-diabetic prescription medication used to treat Type 2 diabetes. It regulates glucose levels and improves the body’s insulin levels to reduce high blood sugar. One possible rare side effect of taking metformin is lactic acidosis, which can be fatal. The risk of lactic acidosis is increased for metformin users if they have kidney problems. Because contrast agents may temporarily decrease kidney function, the risk of lactic acidosis is increased when taking metformin and receiving a contrast dye for a CT scan.

It’s also important to note that metformin can be found in other diabetic medications that use a combination of drugs, like Glucovance, Janumet, Synjardy, and Kazano.

Dr. Youssof explained why the risk of lactic acidosis increases for metformin users taking a CT scan, “The reason is that contrast dye can temporarily affect kidney function, which could interfere with your body’s ability to clear metformin.”

Metformin is cleared through the kidneys. Since there’s a slim chance the contrast dye from the CT scan may cause a contrast-induced acute kidney injury or impaired kidney function, metformin may not be filtered out as it should. If metformin accumulates in the body, there is a higher risk of developing lactic acidosis. Lactic acidosis makes the blood overly acidic, which can lead to fatal complications. 

While lactic acidosis is rare, especially for those with normal renal function, the condition is quite serious. While healthy adults may be fine with intravenous contrast while on metformin, physicians like to use caution, especially for patients over 80 years old. 

A systematic review in 2019 even found that other factors outside of metformin use are more likely to lead to metabolic acidosis after receiving contract agents. This wasn’t even looking at healthy patients but individuals with mild to moderate renal failure. That said, most clinicians still recommend discontinuing metformin before contrast administration. 

There are two types of lactic acidosis: Type-A lactic acidosis and Type-B lactic acidosis. Type-A Lactic Acidosis happens when the body’s tissues do not get adequate oxygen, resulting in the overproduction of lactic acid. Type B includes lactic acidosis that’s not a result of oxygen deprivation. Typically, Metformin-Associated Lactic Acidosis (MALA) falls under the Type-B umbrella.

The signs and symptoms of lactic acidosis can be serious, depending on the severity of your case. Common symptoms include: 

  • Confusion, disorientation
  • Jaundice (yellowing of the skin or eyes)
  • Fast or deep breathing
  • Fast heart rate
  • Muscle cramping
  • Weakness
  • Fatigue
  • Diarrhea
  • Nausea or vomiting 
  • Headaches
  • Reduced appetite

Those with the greatest risk factors of developing lactic acidosis are those with pre-existing kidney problems.

How long do you have to stop metformin before a CT scan?

“Doctors recommend stopping metformin 48 hours before a CT scan with contrast dye,” said Dr. Youssouf. “This allows your body enough time to clear the drug before the procedure.”

Upon taking metformin, it starts taking effect within a few days. The half-life of Metformin is around 20 hours, this means the concentration of the original dose is now at half the dose within the body. Metformin is not metabolized in the liver like many drugs. According to StatPearls, “Instead, [metformin] is primarily eliminated through the kidneys, mostly unchanged. Hence, monitoring renal function is crucial when using metformin to ensure safe and effective treatment.”

In reality, the amount of time metformin will stay in your system differs for everybody. “[Metformin] typically clears from the bloodstream within 48 hours, although the exact timeframe can vary based on factors like kidney function, dosage, and the individual’s metabolic rate,” said Dr. Rohit Vuppuluri, a double-board certified Interventional Cardiologist at Chicago Heart & Vascular Specialists.

While stopping metformin 48 hours before your radiology appointment should suffice for most healthy patients, speaking with your doctor about the proper procedures before your upcoming CT scan is always helpful.

When can you start taking metformin again?

Most individuals can start taking metformin 48 hours after their CT scan. However, your healthcare provider may want to monitor your kidney function to ensure it’s at normal or baseline before resuming the medication.

It’s common for metformin users to undergo kidney function tests after the CT scan to ensure everything has returned to normal. These could include serum creatinine blood tests or estimated glomerular filtration rate (GFR) measurements. 

  • Serum Creatinine Test: Creatinine is waste found in the body and made up of ingested protein and the normal breakdown of muscles. It is filtered through the kidneys, so a serum creatinine test can indicate how well your kidneys filter blood. 
  • GFR Tests: An estimated GFR (eGFR) estimates how well your kidneys are functioning by looking at the amount of creatinine and cystatin C (a type of protein) in your body. For a more accurate measure, a doctor can also request a measured GFR (mGFR), which looks at different agents, but this is more expensive and less readily available.

“It is crucial to wait for this clearance to avoid any complications,” warned Dr. Youssouf. Once you’re given the all-clear, you can start your metformin medication once again.

Metformin guidelines for other types of diagnostic imagining 

“For imaging procedures that do not involve contrast dye—such as MRI without contrast, ultrasound, or X-rays—there are generally no restrictions on taking metformin,” said Dr. Vuppuluri.

Diagnostic imaging procedures that use contrast media include CT scans, magnetic resonance imaging (MRI) with contrast, angiography (X-ray of blood vessels), and venography (X-ray of veins). As these can also have a risk of contrast-induced nephropathy, where renal function deteriorates, they may come with similar metformin guidelines. Before any diagnostic procedure, it’s important to seek a radiologist’s advice on any safeguarding for diabetic patients. 

Another popular radiology test, positron emission tomography (PET) scan, also has special considerations for metformin users. While it uses a different contrast medium (radioactive glucose), studies in recent years have shown that metformin can cause false negatives for cancer detection in the gastrointestinal tract. Due to this, healthcare providers are beginning to recommend patients stop taking metformin 48 to 72 hours before a PET scan.

The bottom line

When scheduled for diagnostic imaging that uses contrast dyes, like a CT scan, it may be helpful for metformin users to stop taking the medication 48 hours before the procedure. While it’s rare, having metformin in your system when given contrast dyes can increase the risk of lactic acidosis. The doctor will often monitor your kidney function through various tests afterward to ensure the vital organ is working properly before resuming metformin.

As with any medical procedure, discuss the guidelines and any concerns you may have with your healthcare practitioner beforehand. Following these rules will help decrease the likelihood of any associated risks.