What the label lists and what reports show
Zanamivir side effects
Zanamivir's FDA label: “The most common adverse events reported in greater than 1.5% of subjects treated with RELENZA and more commonly than in subjects treated with placebo are: • Treatment Trials – sinusitis, dizziness.” FAERS holds 2,524 reports naming it through July 30, 2026; the reaction reported most often, leaving out the conditions it is used for, product-use terms and outcomes, is abnormal behaviour (397 reports).
- No generic listed
Check it at the source: the zanamivir FDA label on DailyMed (effective October 24, 2023, GlaxoSmithKline LLC); the FDA FAERS public dashboard.
What does the patient leaflet say about side effects?
From the patient leaflet: What are the possible side effects of RELENZA?RELENZA can cause serious side effects, including: • Bronchospasm (wheezing). Serious breathing problems, including death, have happened during treatment with RELENZA in people with and without a history of breathing problems. Stop taking RELENZA and get emergency medical help right away if you develop: • worsening breathing • shortness of breath • chest pain or tightness • cough • If you have breathing problems, such as asthma and COPD and your healthcare provider has prescribed RELENZA, you should have a fast‑acting, inhaled bronchodilator available for your use. See “How should I take RELENZA?” • Serious allergic reactions. RELENZA can cause serious allergic reactions. […]
Source: FDA drug label, GlaxoSmithKline LLC, effective October 24, 2023 (SPL set d7c3bcc3-0c0d-4068-fd80-88cf54a376ef), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsThe most common adverse events reported in greater than 1.5% of subjects treated with RELENZA and more commonly than in subjects treated with placebo are: • Treatment Trials – sinusitis, dizziness.
Table 4. Summary of Adverse Events ≥1.5% Incidence during 28-Day Prophylaxis Trials in Adults, Adolescents, and Children a
| Adverse Event | RELENZA (n = 2,231) | Placebo (n = 2,239) |
|---|---|---|
| Headaches | 24% | 26% |
| Throat and tonsil discomfort and pain | 19% | 20% |
| Nasal signs and symptoms | 12% | 13% |
| Ear, nose, and throat infections | 2% | 2% |
| Cough | 17% | 18% |
| Viral respiratory infections | 3% | 4% |
| Muscle pain | 8% | 8% |
| Musculoskeletal pain | 6% | 6% |
| Arthralgia and articular rheumatism | 2% | <1% |
| Feeding problems (decreased or increased appetite and anorexia) | 4% | 4% |
| Nausea and vomiting | 2% | 3% |
| Diarrhea | 2% | 2% |
| Temperature regulation disturbances (fever and/or chills) | 9% | 10% |
| Malaise and fatigue | 8% | 8% |
Rows and numbers as the label prints them, under its own column headings (the first rows of a longer table where it runs on). A reaction in a trial table is not necessarily caused by the medicine, and trials differ from everyday use.
From the label: adverse reactions, continuedThe following adverse reactions are discussed in greater detail in other sections of the labeling: • Bronchospasm [see Warnings and Precautions ( 5.1 )]. • Safety information in patients with underlying airways disease [see Warnings and Precautions ( 5.1 )]. • Allergic-like reactions [see Warnings and Precautions ( 5.2 )]. The most common adverse events reported in greater than 1.5% of subjects treated with RELENZA and more commonly than in subjects treated with placebo are: • Treatment Trials – sinusitis, dizziness. ( 6.1 ) • Prophylaxis Trials – fever and/or chills, arthralgia, and articular rheumatism. ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact GlaxoSmithKline at 1-888-825-5249 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch. 6.1 Clinical Trials Experience Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared with rates in the clinical trials of another drug and may not reflect the rates observed in practice. The placebo used in clinical trials consisted of inhaled lactose powder, which is also the vehicle for the active drug; therefore, some adverse events occurring at similar frequencies in different treatment groups could be related to lactose vehicle inhalation. …
Source: FDA drug label, GlaxoSmithKline LLC, effective October 24, 2023 (SPL set d7c3bcc3-0c0d-4068-fd80-88cf54a376ef), via openFDA; the full label on DailyMed
What do FAERS reports show?
2,524 reports list zanamivir among the medicines taken, each a report of something that happened, not proof the medicine caused it; 250 are coded as a death and 390 as a hospital stay. Counts follow how widely a medicine is used and watched, and they are not a measure of how risky it is. Of the 20 reactions below, 8 are named by the same term (in US spelling) in the label's adverse reactions, warnings or patient leaflet; a reaction the label does not name may still be there under another word, and a report does not show that the medicine caused it.
Reports received each year
The 25 most reported reactions
Source: FDA Adverse Event Reporting System (FAERS) via openFDA, reports through July 30, 2026; reactions as MedDRA preferred terms
What next?
- Zanamivir: the full record
- label, generics, price, shortages, recalls
- Report a side effect to the FDA
- MedWatch
- Side effects hub
- the most reported medicines and reactions