Myocarditis from drugs or vaccines:
when it can occur and which symptoms to recognize

Article written and reviewed by Dr Luca Moretti

Myocarditis caused by a drug or associated with vaccination is rare. There is no single mechanism: some medicines can trigger a hypersensitivity reaction, some cancer therapies can activate an immune response against the heart and, for some vaccines, a small increase in the risk of myocarditis has been documented in specific groups.
The onset of fever, fatigue or muscle aches after a drug or vaccine does not in itself mean that myocarditis is present. The symptoms that require greater attention are mainly chest pain, shortness of breath, palpitations, dizziness or fainting, particularly when they are new, persistent or significant.

When possible myocarditis is being assessed, it is important to reconstruct:

• which drugs were recently started or changed;
• any current cancer or immunological therapies;
• the date of any recent vaccination;
• the exact time when symptoms began;
• any fever, skin rash or other allergic manifestations;
• possible recent infections, which are a common alternative cause of myocarditis.

Myocarditis has many possible causes. Establishing that a drug or vaccine is genuinely responsible therefore requires clinical and temporal assessment, not simply noting that symptoms appeared after an exposure.

When a drug can cause myocarditis

Drug-induced myocarditis is uncommon and can develop through different mechanisms. In some cases, the drug triggers an immune response against cardiac tissue; in others, the injury is related to particular toxic or immunological effects of the treatment. However, not every medicine associated with cardiac injury causes true myocarditis: cardiomyopathies and other forms of cardiotoxicity also exist and must be distinguished from inflammation of the myocardium.
Drugs described in the literature include some antibiotics, neurological and psychiatric drugs, anti-inflammatory drugs and cancer treatments. Clozapine is one of the best-studied examples: associated myocarditis occurs mainly in the first few weeks after treatment is started, which is why some protocols include clinical and laboratory monitoring during the initial phase.

The fact that a medicine has been reported in cases of myocarditis does not mean that the complication is common or that anyone taking it should stop it preventively. When compatible symptoms occur, the doctor assesses the temporal relationship, cardiology test results, other possible triggers and any improvement after the treatment is changed.
It is important not to stop essential medications on your own, especially psychiatric, cancer, immunological or cardiovascular therapies. If suspicion is substantial, the decision to stop or replace the treatment should be made promptly by the doctor managing the therapy together with the specialists involved.

Hypersensitivity myocarditis: when the reaction involves the heart

A particular form is hypersensitivity myocarditis. In this case, the immune system reacts to a drug or, more rarely, another agent, and the inflammation may be characterized by the presence of eosinophils, a type of white blood cell involved in allergic reactions.
Fever, skin rash, increased eosinophils in the blood or other signs of hypersensitivity may occur, but these features are not mandatory. Cardiac presentation can range from mild symptoms to heart failure, arrhythmias or severe forms.

ECG, troponin, echocardiogram and cardiac magnetic resonance imaging can help guide the diagnosis. In eosinophilic forms, or when precisely identifying the type of myocarditis may change treatment, endomyocardial biopsy may be necessary.
Treatment depends on the cause and severity. When a drug reaction is confirmed, discontinuation of the responsible drug is a fundamental part of medical management; in immune-mediated forms, corticosteroids or other immunosuppressive treatments may be used, but these are not treatments to start without a diagnosis and specialist indication.

Cancer immunotherapy: a form that requires particular urgency

Immune checkpoint inhibitors are drugs that allow the immune system to attack certain cancer cells more effectively. In a small percentage of patients, however, they can cause immune-related adverse events, including potentially severe myocarditis. This form deserves particular attention because it can be associated with arrhythmias, electrical conduction disturbances and heart failure that may also progress rapidly.
Immune checkpoint inhibitor myocarditis occurs more often in the first few weeks or months of treatment, although later presentations are also possible.

In a patient receiving cancer immunotherapy, new cardiac symptoms, an increase in troponin, ECG abnormalities or problems such as significant muscle weakness should be reported promptly to the treating team. ESC guidelines recommend an urgent diagnostic pathway in suspected cases and coordinated management involving oncology, cardiology and cardio-oncology.
When the diagnosis is confirmed, treatment may require interruption of immunotherapy and high-dose corticosteroids; in resistant forms, additional immunosuppressive agents may be needed. These are specialist decisions in which cardiac risk and the need for cancer treatment must be balanced.

Myocarditis after a vaccine: how rare it is and when it occurs

Myocarditis can rarely occur after some vaccinations. The best-documented association today concerns COVID-19 vaccines, particularly mRNA vaccines. Surveillance systems have observed a higher frequency especially in adolescent boys and young adult men, typically in the days after vaccination; however, risk is not limited exclusively to this group. Reports and surveillance data have also indicated an increased risk with the protein-based Novavax vaccine.
This does not mean that common post-vaccination arm pain, fever, fatigue or muscle aches represent myocarditis. The main warning sign that should prompt assessment is the onset of chest pain, shortness of breath or palpitations, especially during the first week.

Large population studies have confirmed that post-vaccination myocarditis is a rare event and that risk varies with age, sex, vaccine type and dose. In most cases described after mRNA vaccines, the initial clinical course was favorable, but a diagnosis of myocarditis still requires cardiology follow-up and the usual precautions used for myocardial inflammation.
It is not correct to conclude that every case of myocarditis occurring after vaccination is necessarily caused by the vaccine: recent infections, immune disorders and other causes must be considered. Likewise, previous myocarditis temporally associated with a vaccine does not automatically imply a permanent ban on any future vaccination. The decision depends on the product involved, the probability of a causal relationship, complete recovery and the individual balance of benefits and risks.

Which symptoms to recognize and what to do if myocarditis is suspected

The most important symptoms are the same as in other forms of myocarditis: chest pain, shortness of breath, palpitations, reduced ability to perform usual activities, dizziness or loss of consciousness. After a drug, fever, rash or other signs of an allergic reaction may coexist; during cancer immunotherapy, immune-mediated muscular or neurological disorders may also occur.
Severe or persistent chest pain, significant breathing difficulty, fainting, prolonged palpitations with malaise or rapid deterioration require urgent assessment. In the presence of severe symptoms, it is appropriate to contact emergency services.

The diagnostic pathway may include ECG, troponin and other blood tests, echocardiogram and cardiac magnetic resonance imaging; in selected situations, endomyocardial biopsy may be necessary. It is essential to report accurately all medications, supplements, cancer therapies and recent vaccinations, stating when they were taken and when symptoms began.
The timeline is important, but it is not sufficient on its own to prove causation. The doctor must rule out other conditions, including infections and coronary syndromes, and determine whether the suspected treatment should be stopped, changed or possibly restarted in the future.

Frequently asked questions about myocarditis, drugs and vaccines

Can a drug really cause myocarditis?
Yes, but it is a rare event. Some drugs can trigger an immune or hypersensitivity reaction involving the myocardium; others, such as immune checkpoint inhibitors used in oncology, can cause a specific form of immune-mediated myocarditis.

Should you stop a medication on your own if chest pain develops?
No. Chest pain requires medical assessment, but stopping a treatment on your own can be dangerous. If symptoms are significant or are associated with shortness of breath, palpitations, fainting or marked malaise, urgent medical care should be sought.

Is myocarditis after a vaccine common?
No. Vaccine-associated myocarditis is rare. The best-documented association involves some COVID-19 vaccines, with a higher frequency observed especially in adolescent boys and young adult men in the days after vaccination.

Which symptoms after a drug or vaccine should raise concern for myocarditis?
Chest pain, shortness of breath, palpitations, reduced exercise tolerance, dizziness or fainting require attention. These symptoms do not necessarily indicate myocarditis, but they warrant assessment if they are new or significant.

Does having had myocarditis after a vaccine mean you can never be vaccinated again?
There is no single answer that applies to all vaccines and all situations. The decision about any subsequent doses should be made with a doctor based on the vaccine involved, the probability of a causal relationship, complete resolution of myocarditis and the individual balance of benefits and risks.