Metformin and Contrast Dye: What You Need to Know About Lactic Acidosis and Kidney Risk

Every year, millions of people with type 2 diabetes get an imaging scan-CT, angiogram, or even a simple X-ray with contrast dye. For most, it’s routine. But if you take metformin, you’ve probably heard the warning: stop your pill before the scan. For years, that advice was blanket and rigid. Today, it’s not. The real risk is far smaller than we thought-and stopping metformin unnecessarily can do more harm than good.

Why the Fear Started

Metformin has been the go-to diabetes drug since the 1990s. It’s cheap, effective, and helps people lose weight. But in the 1970s, another biguanide drug, phenformin, was pulled from the market because it caused lactic acidosis-a dangerous buildup of acid in the blood. Metformin was thought to be safer, but doctors still worried. The fear? Contrast dye could hurt your kidneys. If your kidneys slow down, metformin builds up. And if metformin builds up, it might push your body into lactic acidosis.

The theory made sense. But the numbers didn’t back it up.

How Metformin Actually Works-and When It Gets Dangerous

Metformin doesn’t cause lactic acidosis by itself. It works by making your liver less greedy with glucose and helping your muscles use insulin better. But it also slightly interferes with how mitochondria produce energy. That can, in rare cases, nudge cells toward anaerobic metabolism, which produces lactate. Normally, your kidneys clear metformin fast-about 500 ml per minute if your kidneys are healthy. That’s why most people never have a problem.

The danger kicks in when two things happen together: your kidneys are already struggling, and your body is under stress. Think: heart failure, sepsis, dehydration, or liver disease. That’s when metformin sticks around longer, lactate piles up, and acid levels rise. The risk isn’t from the contrast dye. It’s from the combination of poor kidney function + metformin + another illness.

Contrast Dye and Kidneys: The Real Story

Contrast dye used in scans can sometimes cause temporary kidney stress-called contrast-induced acute kidney injury (CI-AKI). But even that’s rarer than people think. In healthy people, it almost never happens. In people with existing kidney disease (eGFR under 30), the risk is higher. But here’s the key: CI-AKI is usually mild and reversible. It doesn’t mean your kidneys are permanently damaged.

The old fear was that contrast dye would crash your kidney function, trap metformin inside you, and trigger lactic acidosis. But studies show that even in patients with mild kidney issues, the actual number of lactic acidosis cases after contrast dye is tiny-fewer than 10 per 100,000 patient-years. That’s less than the risk of being struck by lightning.

What the Guidelines Say Now (2026)

The FDA updated its guidance in 2016. So did the American College of Radiology and the American Diabetes Association. Here’s what they agree on today:

• If your eGFR is above 60 mL/min/1.73 m²: Keep taking metformin. No need to stop.
• If your eGFR is between 30 and 60: Stop metformin at the time of the scan. Restart it 48 hours later-only if your kidney function hasn’t changed.
• If your eGFR is below 30: Don’t take metformin anyway. It’s already contraindicated.
• If you’re getting intra-arterial contrast (like during a heart catheterization): Stop metformin regardless of kidney function. This is the one exception.
• If you have heart failure, liver disease, or are severely dehydrated: Stop metformin before the scan, even if your eGFR is normal.

Why Stopping Metformin Can Be Risky

Many people think stopping metformin is harmless. It’s not. If you stop it cold, your blood sugar can spike within days. For older adults or those with heart disease, that spike raises the risk of stroke, heart attack, or hospitalization. One 2021 study showed that patients who stopped metformin before a CT scan had a 23% higher chance of needing emergency care for high blood sugar within two weeks.

The old rule-stop everything, wait 48 hours-was born from fear, not data. Now we know: the risk of lactic acidosis is so low, it’s outweighed by the risk of uncontrolled diabetes.

Who Still Needs to Be Careful?

You’re at higher risk if you have:

• eGFR between 30 and 60
• Heart failure (especially if you’re on diuretics)
• Chronic liver disease
• Severe infection or sepsis
• Age over 65 with reduced mobility or poor hydration
• Alcohol use disorder

If you fall into one of these groups, your doctor should check your kidney function before the scan. If your eGFR is borderline, they might give you IV fluids before and after the contrast to protect your kidneys. That’s often more helpful than stopping metformin.

What Happens If Lactic Acidosis Does Occur?

It’s rare-but if it happens, it’s serious. Symptoms come on slowly: nausea, vomiting, belly pain, fast breathing, confusion, dizziness. Blood tests show high lactate, low pH, and a wide anion gap. The only real treatment? Dialysis. Hemodiafiltration pulls metformin and lactate out of your blood fast. Survival rates jump from under 60% to over 80% if treatment starts early.

But here’s the thing: most ICU cases happen in people who already had multiple health problems. Not someone with mild kidney decline who got a CT scan.

What You Should Do Before Your Scan

Don’t guess. Don’t assume. Ask your doctor these questions:

1. What’s my latest eGFR? (Make sure it’s from the last 3 months.)
2. Am I getting IV or intra-arterial contrast?
3. Do I have heart failure, liver disease, or an active infection?
4. If I stop metformin, how will you manage my blood sugar?
5. Will you recheck my kidney function 48 hours after the scan?

If your eGFR is above 60 and you’re otherwise healthy, you can probably take your metformin as normal. No need to skip a dose. No need to panic.

The Bigger Picture: Medicine Is Evolving

This isn’t just about metformin. It’s about how medicine changes. For decades, we overreacted to theoretical risks. We stopped medications, delayed procedures, and scared patients with worst-case scenarios. Now, we’re learning to weigh real numbers against real harm.

Metformin remains the most prescribed diabetes drug in the world-over 150 million prescriptions a year in the U.S. alone. If every patient with mild kidney disease stopped it before every scan, we’d be causing more harm than we prevent.

The new guidelines aren’t reckless. They’re precise. They’re based on decades of data, not fear. And they’re saving people from unnecessary hospital visits, blood sugar crashes, and stress.

Final Takeaway

You don’t need to stop metformin before every scan. You only need to stop it if your kidneys are already struggling, or if you’re getting contrast directly into an artery. If you’re healthy, your kidneys are fine, and you’re getting IV contrast-keep taking your pill. Your blood sugar will thank you.

If you’re unsure, ask for your eGFR number. Ask if your scan uses IV or arterial contrast. Ask what the plan is if your kidney function changes. Knowledge beats fear every time.