Most antibiotic warnings concern things you notice during the course: a rash, diarrhea, an upset stomach. Clarithromycin carries a different kind of warning, one that the US Food and Drug Administration (FDA) describes as a possible increased risk of heart problems or death that can show up years after a short course, in people who already have heart disease. On top of that, clarithromycin shares a better-known short-term issue with other macrolides: it can disturb the heart’s electrical rhythm.
This guide unpacks clarithromycin heart risks in a way that separates fact from alarm. We trace the FDA’s actions on a timeline, explain the trial behind the warning in plain numbers, distinguish the short-term rhythm risk from the long-term mortality signal, and give you a practical checklist for your next prescription.
Key takeaways: In 2018 the FDA advised caution before prescribing clarithromycin to people with heart disease and suggested considering other antibiotics. The warning comes mainly from the CLARICOR trial, in which patients with stable coronary heart disease who took clarithromycin for two weeks had more deaths over the following years than those given placebo. Separately, clarithromycin can prolong the QT interval and, rarely, trigger a dangerous rhythm. If you have heart disease, tell every prescriber, but do not stop any medicine on your own.
Two Different Heart Risks, Not One
People often blur these together, but they are separate issues with different timing, mechanisms, and evidence.
| Risk 1: QT prolongation | Risk 2: Long-term mortality in heart disease | |
|---|---|---|
| When it happens | During treatment, usually within days | Observed one year or more after a short course |
| Who is affected | Anyone, but mainly people with extra rhythm risk factors | People with existing coronary heart disease |
| Mechanism | Blocks potassium channels that reset heart cells between beats | Unknown; the FDA says no clear explanation exists |
| Main evidence | ECG studies, case reports, class effect of macrolides | CLARICOR randomized trial plus mixed observational studies |
| Shared by azithromycin? | Yes, azithromycin has its own warning | Not established for azithromycin |
The Regulatory Timeline
| Year | Event |
|---|---|
| 1991 | Clarithromycin (Biaxin) approved in the US. |
| 1990s to 2000s | Reports of QT prolongation and torsades de pointes accumulate, especially with interacting drugs such as cisapride, pimozide, and terfenadine. Labels add contraindications. |
| 2005 | Early CLARICOR results show higher cardiovascular death in clarithromycin-treated heart patients. The FDA issues a safety communication but does not change labeling or make specific recommendations at the time. |
| 2013 | The FDA warns about azithromycin and potentially fatal heart rhythms, drawing attention to macrolides as a class. |
| 2015 | Ten-year CLARICOR follow-up is published in the International Journal of Cardiology. |
| February 22, 2018 | The FDA issues a new Drug Safety Communication advising caution in patients with heart disease, adds a warning and the CLARICOR results to clarithromycin labels, and suggests considering other antibiotics. |
You can read the original FDA Drug Safety Communication and its printable PDF.
What the FDA Actually Said
The 2018 communication made several key points, which we paraphrase here:
- The FDA advises caution before prescribing clarithromycin to patients with heart disease because of a potential increased risk of heart problems or death that can occur years later.
- Prescribers should weigh benefits and risks and consider other available antibiotics for these patients.
- Patients with heart disease should be told about the signs and symptoms of cardiovascular problems, whatever infection they are being treated for.
- The agency could not determine why the risk of death was greater.
- Because there were no long-term randomized trials in people without heart disease, and observational studies were mixed, the FDA could not say whether the CLARICOR findings apply to people without heart disease.
- Patients should not stop their heart medicine or antibiotic without talking to their healthcare professional.
The FDA also noted that clarithromycin is not approved to treat heart disease. That point matters because of how the trial came about.
Inside the CLARICOR Trial
Why it was done
In the 1990s, some researchers suspected that a bacterium called Chlamydophila pneumoniae might contribute to artery inflammation and heart attacks. If so, an antibiotic active against it might protect the heart. Clarithromycin was a logical candidate. The Danish CLARICOR trial set out to test whether a short course could reduce deaths, heart attacks, and unstable angina in people with stable coronary heart disease.
How it worked
About 4,400 adults with stable coronary heart disease were randomly assigned to take either clarithromycin (500 mg once daily) or a placebo for two weeks. Neither patients nor doctors knew who received which. Researchers then followed them for years.
What it found
The hoped-for benefit did not appear. Instead, the clarithromycin group fared worse:
- After an average follow-up of about 2.6 years, all-cause death was 9.8% in the clarithromycin group versus 7.8% with placebo, and cardiovascular death was 5.1% versus 3.4%, according to a summary by the Nebraska Medicine antimicrobial stewardship program.
- Over 10 years of follow-up, clarithromycin was associated with a trend toward higher all-cause mortality (hazard ratio about 1.10, a roughly 10% relative increase) and a significant increase in cerebrovascular disease such as stroke (hazard ratio about 1.19).
- The excess risk was concentrated in patients who were not taking statins when they entered the study.
How to read those numbers
A hazard ratio of 1.10 means the event rate was about 10% higher in relative terms, not that 10% more people died. In absolute terms, the difference over the early follow-up was about two extra deaths per 100 patients. That is meaningful at a population level, and it came from a randomized trial, which is the strongest type of evidence for cause and effect. But the trial tested a population with established coronary disease in the early 2000s, before modern standards of statin use, so how it applies to today’s patients is uncertain.
The observational evidence
The FDA also reviewed six observational studies in people with or without coronary disease. Two found evidence of long-term risk and four did not. Observational studies are prone to bias (for example, sicker people may be more likely to be prescribed certain antibiotics), so the FDA regarded CLARICOR as the strongest evidence.
Why might a two-week antibiotic matter years later?
Honestly, no one knows. The FDA states that the mechanism is unexplained. Researchers have speculated about effects on inflammation, plaque stability, or interactions with other heart medicines, and the statin finding has prompted ideas that statins somehow buffer the effect. None of these has been proven. The uncertainty is part of why the warning is framed as caution rather than a ban.
The Short-Term Risk: QT Prolongation Explained
Each heartbeat on an electrocardiogram (ECG) includes a stretch called the QT interval, which reflects the time the heart’s lower chambers take to electrically “recharge” after contracting. Clarithromycin, like other macrolides, can slow this recharging by blocking specific potassium channels. A longer QT on its own causes no symptoms. The concern is that, in vulnerable people, it can allow a chaotic rhythm called torsades de pointes, which can cause fainting or sudden cardiac arrest.
The US label states that cases of torsades de pointes have been reported after marketing and advises avoiding clarithromycin in people with:
- Known QT prolongation or a history of ventricular arrhythmia, including torsades.
- Uncorrected low potassium (hypokalemia) or low magnesium (hypomagnesemia).
- Clinically significant slow heart rate (bradycardia).
- Use of class IA antiarrhythmics (quinidine, procainamide) or class III antiarrhythmics (amiodarone, dofetilide, sotalol), or other drugs known to prolong QT.
The label adds that older adults may be more susceptible. Other recognized risk factors across QT-prolonging drugs include female sex, heart failure, kidney or liver impairment (which raise drug levels), and congenital long QT syndrome.
Interactions that amplify the risk
Because clarithromycin blocks the CYP3A4 enzyme, it can raise blood levels of other QT-prolonging drugs, creating a double hit. Pimozide and cisapride are contraindicated for this reason. Quetiapine, some antipsychotics, methadone, and certain antifungals deserve extra caution. Clarithromycin can also raise digoxin levels and, with calcium channel blockers, has been linked to low blood pressure and kidney injury in older adults. See our clarithromycin drug interactions guide for the full list.
Reading your ECG numbers
If your doctor orders an ECG, the key figure is the QTc, the QT interval corrected for heart rate. Exact cut-offs vary by source, but commonly used reference points are:
| QTc | General interpretation |
|---|---|
| Under about 450 ms (men) or 460 ms (women) | Normal range |
| About 450 to 500 ms | Borderline to prolonged; QT-prolonging drugs used cautiously or avoided |
| Above 500 ms, or an increase of more than 60 ms from baseline | High-risk zone; most clinicians stop or avoid QT-prolonging drugs |
Your clinician interprets these values alongside symptoms, electrolytes, and other medicines. A single number is not a diagnosis.
Why women and older adults are more vulnerable
Women naturally have slightly longer QT intervals than men, and most reported cases of drug-induced torsades occur in women. Older adults more often have reduced kidney function, slower heart rates, heart disease, and multiple medicines, all of which add risk. Neither group needs to avoid clarithromycin automatically, but both benefit from a careful medication review.
Putting the Risk in Perspective
Warnings like these can sound frightening. A few points help keep them in proportion:
- Torsades de pointes from clarithromycin is rare. Most cases involve multiple risk factors at once, such as an interacting drug plus low potassium.
- The CLARICOR signal comes from people with established coronary disease. It may not apply to healthy adults, and the FDA explicitly said it could not determine that.
- Absolute differences were modest. Meaningful for public health and prescribing policy, but not a reason for panic if you took a course in the past.
- Untreated infections are dangerous too. Pneumonia itself strains the heart and raises the short-term risk of heart attacks. The goal is choosing the safest effective antibiotic, not avoiding treatment.
Questions to Ask Your Prescriber
- Given my heart history, is there an equally effective antibiotic without these warnings?
- Do I need an ECG before starting, or during treatment?
- Should my potassium or magnesium be checked?
- Do any of my current medicines raise my QT risk or interact with clarithromycin?
- Is my dose adjusted for my kidney function?
- Which symptoms should make me stop and seek help immediately?
How Clarithromycin Compares With Other Antibiotics
| Antibiotic | QT effect | Long-term heart signal | CYP3A4 interactions |
|---|---|---|---|
| Clarithromycin | Yes | FDA caution in heart disease (CLARICOR) | Strong |
| Azithromycin | Yes; FDA warning 2013 | Not established | Minimal |
| Erythromycin | Yes, notably with IV use | Not established | Strong |
| Fluoroquinolones (levofloxacin, moxifloxacin) | Yes | Separate warnings (aortic aneurysm) | Minimal |
| Doxycycline | No meaningful effect | None known | Minimal |
| Amoxicillin / amoxicillin-clavulanate | No meaningful effect | None known | Minimal |
Azithromycin is often suggested as an alternative, but it is not risk-free for rhythm. Read azithromycin and heart rhythm and azithromycin vs clarithromycin to understand the trade-offs. For many infections, amoxicillin or doxycycline avoids both concerns; see clarithromycin vs amoxicillin.
A Note on Azithromycin’s Own Heart Warning
Because azithromycin is the usual macrolide alternative, it is worth knowing why it has a warning too. In 2012, a large study of Tennessee Medicaid patients published in the New England Journal of Medicine found a small increase in cardiovascular deaths during five-day azithromycin courses compared with amoxicillin or no antibiotic. The extra risk was concentrated in people who already had high cardiovascular risk. That study helped prompt the FDA’s 2013 azithromycin warning. The key difference from clarithromycin is timing: azithromycin’s signal was seen during treatment, consistent with a rhythm effect, while clarithromycin’s CLARICOR signal emerged over the following years. Neither finding means these drugs are dangerous for most people, but both argue for thoughtful antibiotic choice in heart patients.
What This Means for You: A Risk Checklist
Before taking clarithromycin, go through this list and share any “yes” answers with your prescriber or pharmacist.
Long-term risk factors (CLARICOR-related)
- Have you been diagnosed with coronary artery disease, angina, or a previous heart attack?
- Have you had a stent, bypass surgery, or a stroke or mini-stroke?
- Do you have heart failure?
- Are you not currently on a statin despite having heart disease?
Short-term rhythm risk factors (QT-related)
- Have you ever been told you have a long QT interval or an abnormal ECG?
- Have you fainted unexpectedly, or does anyone in your family have a history of sudden unexplained death at a young age?
- Do you take an antiarrhythmic, antipsychotic, certain antidepressants, methadone, ondansetron, or another QT-prolonging drug?
- Do you have low potassium or magnesium, often from diuretics, vomiting, or diarrhea?
- Is your heart rate unusually slow?
- Do you have significant kidney or liver disease?
- Are you over 65?
What your prescriber might do
- Choose a different antibiotic that treats your infection equally well.
- Check an ECG before or during treatment.
- Check potassium and magnesium and correct them if low.
- Adjust or pause interacting medicines.
- Use the shortest effective course and the correct dose for your kidney function.
Some situations still justify clarithromycin in people with heart disease, for example certain mycobacterial infections, specific H. pylori regimens guided by susceptibility testing, or allergies that rule out alternatives. The FDA’s message is to weigh the decision, not to forbid it.
The Statin Puzzle
One of the most intriguing CLARICOR findings was that the long-term excess risk was confined to participants who were not taking a statin at the start of the study. Several explanations have been suggested. Statins stabilize artery plaque and reduce inflammation, which might counteract whatever harmful effect clarithromycin has. Alternatively, patients on statins in the early 2000s may have differed in other ways, such as receiving more attentive heart care.
Two cautions follow. First, this was a subgroup finding, which makes it less certain than the main result. Second, it does not mean you should start a statin just to take clarithromycin, nor that being on a statin makes clarithromycin risk-free. Remember too that clarithromycin interacts strongly with simvastatin, lovastatin, and atorvastatin, so the statin you take matters for short-term safety. Details are in our drug interactions guide.
When the Benefits Can Outweigh the Risks
The FDA framed its advice as a balance, and there are situations where clarithromycin remains the right drug even for someone with heart disease:
- Mycobacterium avium complex and other nontuberculous mycobacteria: macrolides are the cornerstone of therapy, and azithromycin may not always be suitable.
- H. pylori infection with a proven susceptible strain, when other regimens are unsuitable because of allergy or prior failure.
- Penicillin allergy plus interactions or QT concerns with other options, where clarithromycin is the least risky choice available.
- Specific infections where culture results show clarithromycin is one of the few active drugs.
In these cases, the practical steps are to use the shortest effective course, correct electrolytes, check an ECG if indicated, adjust interacting drugs, and make sure the patient knows which symptoms to report.
Symptoms That Need Immediate Attention
Call 911 or go to the emergency department if, during or after a clarithromycin course, you experience:
- Chest pain, pressure, or tightness.
- Fainting or near-fainting, especially without warning.
- A fast, pounding, fluttering, or irregular heartbeat that does not settle.
- Sudden shortness of breath.
- Weakness, numbness, or drooping on one side of the face or body.
- Slurred speech or sudden confusion.
- Seizure.
These match the warning signs the FDA highlighted for heart attack and stroke. Tell emergency staff that you are taking or recently took clarithromycin and list your other medicines.
If You Have Already Taken Clarithromycin
If you have heart disease and took clarithromycin in the past, there is no need to panic. The increased risk seen in CLARICOR was modest in absolute terms, and many observational studies did not find it. The most useful actions are the same ones that protect your heart anyway:
- Take your heart medicines as prescribed, including statins if they are recommended for you.
- Keep blood pressure, cholesterol, and blood sugar under control.
- Do not smoke, stay active within your limits, and attend regular check-ups.
- Make sure clarithromycin and any other macrolide reactions are recorded in your medical record.
Our guide to Adderall and heart health includes practical home blood pressure monitoring tips that apply to anyone tracking cardiovascular risk, and men managing heart disease alongside ED treatment can find relevant guidance in Cialis and heart health.
Help for Caregivers
If you look after an older relative with heart disease, you are often the person who notices problems first. During a clarithromycin course, keep an eye out for new dizziness, unusual tiredness, confusion, swelling, reduced urination, or complaints of a fluttering heartbeat. Keep an updated medication list handy, including any medicines paused for the antibiotic course, and make sure every prescriber, including urgent care or dental clinics, knows about the heart condition before an antibiotic is chosen.
Reporting Side Effects
If you experience a heart-related problem after clarithromycin, you or your clinician can report it to the FDA’s MedWatch program, as noted on MedlinePlus. Reports help the agency continue monitoring this issue, which it said it would do.
Common Questions
Does the FDA say clarithromycin is banned for heart patients?
No. It is not banned or contraindicated for heart disease in general. The FDA advises caution, weighing benefits and risks, and considering alternatives.
Does the warning apply if I do not have heart disease?
The FDA said it could not determine whether the CLARICOR results apply to people without heart disease. The short-term QT precautions apply to everyone.
Is azithromycin safer for my heart?
Azithromycin has not shown the same long-term mortality signal and has far fewer drug interactions, but it carries its own QT warning. For some infections, non-macrolide antibiotics may be preferable.
Should I stop clarithromycin if I have heart disease?
Do not stop on your own. Call your prescriber, who can decide whether to continue, switch, or monitor you.
The Bottom Line
Clarithromycin carries two distinct heart concerns: a short-term risk of rhythm disturbances that depends heavily on your other medicines and electrolytes, and a long-term mortality signal in people with coronary heart disease that led the FDA to recommend caution in 2018. Neither makes clarithromycin forbidden, but both make it worth a conversation. If you have any heart condition, say so every time an antibiotic is prescribed, ask whether an alternative would work just as well, and seek help immediately for chest pain, fainting, or palpitations. To understand what the drug treats and why it is still used, see our clarithromycin uses guide.
This article is educational and does not replace advice from your doctor, cardiologist, or pharmacist.
