When you’re prescribed an antibiotic like azithromycin or clarithromycin for a stubborn chest infection, you probably don’t think about your heart. But here’s the thing: these common drugs can mess with your heart’s electrical rhythm. Not often. Not in everyone. But enough that doctors need to know who’s at real risk - and who doesn’t need an ECG before starting treatment.

Why Macrolides Can Be Risky for Your Heart

Macrolide antibiotics - azithromycin, clarithromycin, erythromycin - work great against pneumonia, bronchitis, and sinus infections. But they also block a specific potassium channel in heart cells called hERG. That’s not a problem for most people. But for some, it slows down the heart’s recovery phase after each beat. On an ECG, that shows up as a longer QT interval. And when that interval gets too long, it can trigger a dangerous rhythm called Torsades de Pointes - a type of ventricular arrhythmia that can lead to sudden cardiac arrest.

The risk isn’t huge overall. About 1 to 8 cases of Torsades happen per 10,000 people taking macrolides each year. But that number jumps sharply if your QTc (corrected QT interval) is above 500 milliseconds. For every 10 ms above that, your risk goes up by 5-7%. Erythromycin carries the highest risk, followed by clarithromycin. Azithromycin is lower, but still not zero - especially if you’re already at risk.

Who Should Get an ECG Before Taking Macrolides?

The British Thoracic Society says: everyone getting long-term macrolide therapy - think months for bronchiectasis or cystic fibrosis - should have a baseline ECG. That’s because these patients are often on the drug for weeks or longer, and the risk builds up over time.

But here’s the real question: what about someone getting a 5-day course of azithromycin for a bad cough? Do they need an ECG too?

The answer isn’t yes or no - it’s who. According to experts, you need an ECG if you have any of these risk factors:

• Female sex - women are nearly three times more likely to develop drug-induced QT prolongation
• Age 65 or older - risk doubles after 65
• Already taking other QT-prolonging drugs - like certain antidepressants, antifungals, or antiarrhythmics
• History of heart disease, heart failure, or prior arrhythmia
• Low potassium or magnesium levels - common in people on diuretics or with eating disorders
• Chronic kidney disease - especially stage 3 or worse
• Family history of sudden cardiac death or Long QT Syndrome
• Already have a QTc over 450 ms in men or 470 ms in women

That’s it. You don’t need an ECG just because you’re getting a macrolide. But if you have one or more of these, you absolutely should.

What Do Guidelines Actually Say?

There’s a big gap between what guidelines recommend and what actually happens in clinics.

The British Thoracic Society (BTS) has clear rules: baseline ECG before long-term macrolide use, then another at one month. In specialized respiratory clinics in the UK, 87% follow this. But in regular GP offices? Only 12% do.

Why? Time. Cost. Uncertainty.

An ECG costs around £28.50 in the UK. With over 12 million macrolide prescriptions issued annually, universal screening would cost £342 million a year. That’s not feasible. And most GPs aren’t trained to interpret borderline QTc values - say, 470-499 ms - which are tricky. One misread, and you might miss a dangerous case.

Meanwhile, the American Heart Association and NIH now support a risk-stratified approach. That means you don’t screen everyone - you screen the people who are most likely to have problems. The 2025 update to the AHA guidelines introduced a 9-point scoring system that weighs age, sex, kidney function, and drug interactions. If your score is 4 or higher, get the ECG. If it’s lower, you’re probably fine.

What Happens If You Skip the ECG?

Most people will be fine. But some won’t.

There’s a documented case from a Reddit thread in March 2025: a 68-year-old woman with no known heart issues, QTc of 480 ms at baseline, took clarithromycin for pneumonia. Five days later, she went into Torsades de Pointes. Emergency cardioversion saved her life. She had never had an ECG before. Her doctor assumed she was healthy - and she was, except for one hidden risk: she was on a diuretic and had borderline low potassium.

That’s the problem. Risk factors aren’t always obvious. A patient might not mention their old diuretic. Or they might not know their dad died suddenly at 52. That’s why screening isn’t about suspicion - it’s about catching the silent risks.

On the flip side, mandatory screening in respiratory clinics found that 1.2% of patients had previously undiagnosed Long QT Syndrome. That’s one in 80 people. Without screening, they’d have been prescribed macrolides - and might not have survived.

How Is This Done in Practice?

If you’re in a hospital or a specialist clinic, it’s straightforward. The ECG is ordered before the prescription is written. Results come back in minutes. If QTc is over 450 (men) or 470 (women), the doctor switches to a safer antibiotic - like amoxicillin or doxycycline.

In primary care? It’s messier.

A 2024 survey of 247 UK GPs showed 78% knew macrolides could prolong QT. But only 22% ordered baseline ECGs. Why? 65% said they didn’t have time. 58% said guidelines weren’t clear for short courses. 47% thought healthy patients were fine.

But here’s what’s changing: electronic health records are starting to help. Epic Systems, one of the biggest EHR platforms in the US, now flags macrolide prescriptions if the patient has risk factors - like age, kidney disease, or another QT-prolonging drug. That alert pops up right when the doctor clicks “prescribe.” It doesn’t force a decision, but it reminds them: Did you check the ECG?

In the UK, pilot programs are testing point-of-care ECG devices in respiratory clinics. These handheld machines give results in under 2 minutes. One clinic reduced the wait time for treatment from 5.2 days to 0.8 days. That’s huge.

What If Your QTc Is Borderline?

Let’s say your ECG shows a QTc of 465 ms. You’re a 52-year-old woman. No heart disease. No other meds. Just a simple chest infection.

Here’s what to do:

1. Check your electrolytes - get a simple blood test for potassium and magnesium.
2. Review all your other medications - even over-the-counter ones. Antihistamines like diphenhydramine can also prolong QT.
3. Consider switching to a non-macrolide antibiotic if possible.
4. If you must use a macrolide, avoid high doses. Azithromycin 500 mg once daily for 5 days is safer than 1 g on day one.
5. Ask for a repeat ECG after 5-7 days if you’re on the drug longer than a week.

Don’t panic. But don’t ignore it either.

The Bottom Line

ECG monitoring isn’t needed for everyone on macrolides. But it’s critical for people with risk factors. The data is clear: age, sex, kidney function, other drugs, and existing heart conditions make the difference between a safe course of antibiotics and a life-threatening event.

Doctors aren’t being careless. They’re overwhelmed. But technology - automated alerts, point-of-care ECGs, risk scores - is making it easier to do the right thing without burning out.

If you’re prescribed azithromycin or clarithromycin, ask: Do I have any of these risk factors? If yes, push for an ECG. If you’re a clinician, don’t wait for guidelines to catch up. Use the 9-point risk score. Check the meds. Check the labs. It takes two minutes. And it could save a life.