The pharmacological cause: more than a flu bug
It is a common misconception that feeling unwell after an infusion is a sign of catching a virus. In fact, many flu-like symptoms—including fever, chills, body aches, and fatigue—are a well-documented side effect of intravenous medication. These reactions are known as infusion-related reactions (IRRs). The underlying cause is often not infectious but is a systemic inflammatory response triggered by the medication itself.
Cytokine release syndrome (CRS)
One of the most significant pharmacological mechanisms behind IRRs is cytokine release syndrome (CRS). This occurs when immune cells are activated and release large amounts of small protein messengers called cytokines into the bloodstream. A rapid, widespread surge of these inflammatory proteins can lead to a range of symptoms that mimic a severe flu, including fever, headache, and joint and muscle pain. While mild CRS is manageable, severe cases can affect multiple organs and, though rare, can be life-threatening.
Non-immune mechanisms
Not all IRRs are immune-mediated. For some medications, such as intravenous iron, the reaction is thought to be caused by the rapid release of unbound, or labile, iron into the system. This interaction can trigger a temporary inflammatory response that manifests as flu-like symptoms. The timing and intensity of this reaction can vary depending on the specific iron formulation used and the patient's individual risk factors.
Common medications that cause flu-like symptoms
Several classes of medications are known to cause IRRs. Knowing which ones are associated with this side effect can help patients and caregivers prepare and manage expectations.
Intravenous (IV) iron preparations
IV iron is a common cause of delayed, flu-like reactions. Studies have noted that up to a third of patients receiving infusions like ferric carboxymaltose may experience symptoms 2 to 5 days after treatment. These reactions typically include muscle aches, bone pain, and mild fever, lasting 24 to 48 hours.
Bisphosphonates
Medications such as zoledronic acid (Reclast) used to treat osteoporosis are frequently associated with flu-like symptoms. These side effects, including fatigue, fever, and body aches, often occur within 3 days of the infusion and are most common after the first dose. They usually resolve within a few days.
Monoclonal antibodies (mAbs) and immunotherapies
Monoclonal antibodies and other immunotherapies used in cancer and autoimmune disease treatment are particularly known for causing IRRs, often due to CRS. Rituximab, an anti-CD20 antibody, is one example, with reactions most common during the initial infusions. These symptoms can include fever, chills, rigors, and other systemic effects.
Other infused medications
Other drugs, such as the antibiotic vancomycin, can also cause pseudo-allergic reactions that involve flushing and hypotension, often feeling flu-like, if infused too quickly.
Comparison of infusion reactions and allergic reactions
Distinguishing between a standard IRR and a true allergic reaction is critical for proper management.
| Characteristic | Infusion Reaction (Non-Allergic) | Allergic Reaction (Anaphylaxis) |
|---|---|---|
| Cause | Release of inflammatory mediators (e.g., cytokines) due to drug properties, not specific IgE antibodies. | IgE-mediated immune response to the drug as an allergen. |
| Onset | Can be immediate, hours after, or delayed by days. | Rapid onset, typically within minutes of exposure. |
| Immune Response | Innate, non-specific immune activation. | Adaptive, specific IgE antibody response. |
| Symptoms | Fever, chills, fatigue, muscle aches, headache, flushing, nausea, rash. | Hives, severe facial/throat swelling, difficulty breathing, wheezing, hypotension. |
| Recurrence | Often decreases with subsequent infusions as the body builds tolerance. | Risk of severe reaction increases with re-exposure. |
Managing and preventing post-infusion symptoms
For many patients, IRRs can be effectively managed with preparation and supportive care.
Premedication
Healthcare providers may administer premedications before an infusion to reduce the risk of a reaction. Common examples include:
- Antipyretics: Acetaminophen (Tylenol) can reduce fever and body aches.
- Antihistamines: Diphenhydramine (Benadryl) can help with itching or flushing.
- Corticosteroids: Steroids like dexamethasone are used for more severe risks, especially with certain immunotherapies.
Hydration and rest
Adequate hydration is crucial both before and after an infusion. Staying well-hydrated can help alleviate dizziness, fatigue, and nausea. Getting plenty of rest is also essential, as the body expends energy to process the medication.
Infusion rate adjustment
If a mild or moderate IRR occurs during the procedure, the healthcare team may temporarily pause or slow the infusion rate to allow the patient's body to adjust.
When to seek medical attention
While most post-infusion symptoms are mild and resolve on their own, certain signs warrant immediate medical attention, as they could indicate a more severe allergic reaction. Contact your healthcare team or seek emergency services if you experience:
- Trouble breathing or shortness of breath
- Severe facial or throat swelling
- Chest pain or tightness
- Rapid or irregular heartbeat
- Lightheadedness, dizziness, or fainting
- A fever that is high or does not resolve
Conclusion
Yes, it is possible and quite common to get flu-like symptoms after an infusion with certain medications, primarily due to a non-viral inflammatory response in the body. While these infusion-related reactions (IRRs) can be uncomfortable, they are typically mild, self-limiting, and can often be prevented or managed with premedication, proper hydration, and rest. The key is to understand the difference between a mild IRR and a severe allergic reaction, and to always communicate any symptoms to your healthcare team. Recognizing these symptoms and knowing how to respond is vital for a safe and effective treatment experience. For more information, you can read about Infusion or Immune Reactions on the American Cancer Society website.
