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Relenza Side Effects

Generic Name: zanamivir

Note: This document contains side effect information about zanamivir. Some of the dosage forms listed on this page may not apply to the brand name Relenza.

For the Consumer

Applies to zanamivir: inhalation disk

Along with its needed effects, zanamivir (the active ingredient contained in Relenza) may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur while taking zanamivir:

  • Convulsions
  • dizziness and fainting
  • fast, slow, or irregular heartbeat
  • flushing or reddening of the skin
  • increased sensitivity to sunlight
  • itching, pain, redness, swelling, or watering of the eye or eyelid
  • joint pain
  • severe skin rash or hives
  • shortness of breath or troubled breathing
  • swelling or puffiness of the face
  • swollen glands or tightness in the throat
  • tightness in the chest or wheezing

Some side effects of zanamivir may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects. Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

Less common
  • Change in hearing
  • cough
  • cough producing mucus
  • diarrhea
  • dizziness
  • ear drainage
  • ear, nose, and throat infections
  • earache
  • fever or chills
  • headache
  • nasal signs and symptoms
  • nausea
  • pain and pressure over cheeks
  • pain in the ear
  • shortness of breath
  • vomiting

For Healthcare Professionals

Applies to zanamivir: inhalation powder


Zanamivir is generally well-tolerated. In clinical trials, the incidence of adverse events in patients treated with zanamivir (the active ingredient contained in Relenza) was similar to that in the placebo (lactose vehicle) group.[Ref]


In a Phase I study, bronchospasm was reported in 1 of 13 patients with mild or moderate asthma following the administration of zanamivir (the active ingredient contained in Relenza) In a Phase III study of patients with acute influenza-like illness and underlying asthma or chronic obstructive pulmonary disease, more patients on zanamivir (10%) than on placebo (9%) experienced greater than 20% decline in FEV1 after 5 days of treatment.[Ref]

Respiratory side effects have included cough (up to 17%), viral respiratory infections (up to 13%), nasal signs and symptoms (up to 12%), sinusitis (up to 3%), bronchitis (2%), ear, nose and throat infections (up to 2%), nasal inflammation (1%), decline in FEV1, congestion, rhinitis, and dry throat. Asthma, exacerbation of asthma, bronchospasm, respiratory arrest, dyspnea, lung tightness, and/or decline in lung function may occur in patients with underlying respiratory disease treated with zanamivir. Bronchospasm and dyspnea have been reported during postmarketing experience.[Ref]

Nervous system

Nervous system side effects have included headache (up to 24%) and dizziness (up to 2%). Seizures, syncope, and vasovagal-like episodes shortly after inhalation have been reported during postmarketing experience.[Ref]


Other side effects have included throat and tonsil discomfort and pain (up to 19%), malaise (up to 8%), fatigue (up to 8%), temperature regulation disturbances (fever and/or chills; up to 9%), and abdominal pain in less than 1.5% of patients. Facial edema has been reported during postmarketing experience.[Ref]


Musculoskeletal side effects have included muscle pain (up to 8%), musculoskeletal pain (6%), arthralgia and articular rheumatism (up to 2%), and myalgia (less than 1.5%).[Ref]


Dermatologic side effects have included urticaria in less than 1.5% of patients. Rash, including serious cutaneous reactions (e.g., erythema multiforme, Stevens-Johnson syndrome, toxic epidermal necrolysis), and urticaria have been reported during postmarketing experience.[Ref]


Hypersensitivity side effects have included allergic or allergic-like reactions, including oropharyngeal edema, serious skin rashes, and anaphylaxis during postmarketing experience.[Ref]


Psychiatric side effects have included delirium, including symptoms such as altered level of consciousness, confusion, abnormal behavior, delusions, hallucinations, agitation, anxiety, nightmares, during postmarketing experience.

Abnormal behavior and delirium leading to injury, with fatal outcomes in some cases, with the use of zanamivir in influenza patients have been reported. Although frequency is unknown, based on zanamivir usage, these events appear to be uncommon. These events were primarily reported in pediatric patients and often had an abrupt onset and rapid resolution. Since influenza may be associated with neurologic and behavioral symptoms (including hallucinations, delirium, and abnormal behavior, with fatal outcomes in some cases), zanamivir's contribution to these events has not been established.


Gastrointestinal side effects have included diarrhea (up to 3%), nausea (up to 3%), and vomiting (up to 2%).[Ref]


Metabolic side effects have included feeding problems (decreased or increased appetite and anorexia; up to 4%) and elevated creatine phosphokinase.[Ref]


Hepatic side effects have included elevated liver enzymes.[Ref]


Hematologic side effects have included lymphopenia and neutropenia.[Ref]


Cardiovascular side effects have included arrhythmias during postmarketing experience.[Ref]


Renal side effects have included one case report acute renal failure, increased serum creatinine, hyperkalemia, and hypotension in a patient with multiple medical problems and multiple other medications.[Ref]


1. "Product Information. Relenza (zanamivir)" Glaxo Wellcome, Research Triangle Pk, NC.

2. Cheer SM, Wagstaff AJ "Zanamivir: an update of its use in influenza." Drugs 62 (2002): 71-106

3. "Antiviral drugs for influenza." Med Lett Drugs Ther 54 (2012): 1-3

4. Laforce C, Man CY, Henderson FW, et al. "Efficacy and safety of inhaled zanamivir in the prevention of influenza in community-dwelling, high-risk adult and adolescent subjects: a 28-day, multicenter, randomized, double-blind, placebo-controlled trial." Clin Ther 29 (2007): 1579-1590

5. Montalto NJ, Gum KD, Ashley JV "Updated treatment for influenza A and B." Am Fam Physician 62 (2000): 2467-76

6. Moscona A "Neuraminidase inhibitors for influenza." N Engl J Med 353 (2005): 1363-73

7. Williamson JC, Pegram PS "Respiratory distress associated with zanamivir." N Engl J Med 342 (2000): 661-2

8. Jefferson T, Demicheli V, Rivetti D, Jones M, Di Pietrantonj C, Rivetti A "Antivirals for influenza in healthy adults: systematic review." Lancet 367 (2006): 303-13

9. "Antiviral drugs for influenza 2012-2013." Med Lett Drugs Ther 54 (2012): 97-8

10. Macdonald L "New influenza drugs zanamivir (Relenza (TM)) and oseltamivir (Tamiflu (TM)): unexpected serious reactions." Can Med Assn J 163 (2000): 879-81

11. Dreitlein WB, Maratos J, Brocavich J "Zanamivir and oseltamivir: Two new options for the treatment and prevention of influenza." Clin Ther 23 (2001): 327-55

12. Anekthananon T, Pukritayakamee S, Ratanasuwan W, et al. "Oseltamivir and inhaled zanamivir as influenza prophylaxis in Thai health workers: a randomized, double-blind, placebo-controlled safety trial over 16 weeks." J Antimicrob Chemother 68 (2013): 697-707

13. McNicholl IR, McNicholl JJ "Neuraminidase inhibitors: zanamivir and oseltamivir." Ann Pharmacother 35 (2001): 57-70

14. Monto AS, Fleming DM, Henry D, et al. "Efficacy and safety of the neuraminidase inhibitor zanamivir in the treatment of influenza A and B virus infections." J Infect Dis 180 (1999): 254-61

Some side effects of Relenza may not be reported. Always consult your doctor or healthcare specialist for medical advice. You may also report side effects to the FDA.

Disclaimer: Every effort has been made to ensure that the information provided is accurate, up-to-date and complete, but no guarantee is made to that effect. In addition, the drug information contained herein may be time sensitive and should not be utilized as a reference resource beyond the date hereof. This material does not endorse drugs, diagnose patients, or recommend therapy. This information is a reference resource designed as supplement to, and not a substitute for, the expertise, skill , knowledge, and judgement of healthcare practitioners in patient care. The absence of a warning for a given drug or combination thereof in no way should be construed to indicate safety, effectiveness, or appropriateness for any given patient. does not assume any responsibility for any aspect of healthcare administered with the aid of materials provided. The information contained herein is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. If you have questions about the substances you are taking, check with your doctor, nurse, or pharmacist.