Oseltamivir or Zanamivir

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Oseltamivir or Zanamivir


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Oseltamivir is taken as a pill twice each day for 5 days.

Zanamivir is inhaled through a device called a Diskhaler. This inhaler device delivers the medicine to the lungs, where the influenza (flu) virus multiplies. It is inhaled twice a day for 5 days.

How It Works

Zanamivir and oseltamivir are medicines known as neuraminidase inhibitors. They help prevent influenza A and influenza B viruses from multiplying in the body by interfering with the production and release of virus from cells that line the respiratory tract. This may slow the spread of the infection within the airways and lungs.

Why It Is Used

Zanamivir and oseltamivir may be used to treat flu caused by both the influenza A and B viruses.

  • Both medicines are given twice a day for 5 days.
  • Zanamivir is used for people age 7 and older.
  • Oseltamivir is used to treat flu in people age 1 or older.

Health Canada has also approved oseltamivir to prevent flu in adults and children age 1 and older. Zanamivir is approved for use to prevent the flu in adults and children age 7 and older.

How Well It Works

Both zanamivir and oseltamivir have been shown to effectively treat and prevent the flu.1 Using one of these medicines typically shortens the course of influenza A or B by at least 1 day.1, 2

A well-controlled study found that oseltamivir taken within 36 hours of the first flu symptoms reduced the length of illness by 30%, the severity by 40%, and the time to resume normal activities by 2 or 3 days. These results are compared with people who had treatment with a placebo.3

Zanamivir has been shown to prevent flu among household members when a member of the family is infected.4

Zanamivir and oseltamivir may not be as effective for people who do not have a fever or do not have severe flu symptoms. And more study is needed to find out how well these medicines work to prevent the flu in people who have severely impaired immune systems.

Side Effects

Some minor side effects of zanamivir and oseltamivir include:

  • Diarrhea.
  • Nausea and vomiting. This side effect tends to occur with oseltamivir. Taking the medicine with food may reduce the risk.
  • Swelling of the sinuses (sinusitis).

A small number of people, especially children, have shown behaviour changes after taking oseltamivir or zanamivir.5, 6 These include:

  • Attempts to hurt themselves.
  • Confusion or delirium.

Anyone who takes Tamiflu or Relenza, especially children, should be watched closely. If the person seems confused, tries to hurt himself or herself, or is acting strangely, call a doctor right away.

See Drug Reference for a full list of side effects. (Drug Reference is not available in all systems.)

What To Think About

Zanamivir and oseltamivir work best when started within 48 hours of your first flu symptoms.

Some people who have asthma or chronic obstructive pulmonary disease (COPD) and use zanamivir may experience reduced lung function and have trouble breathing. If you have these conditions, talk to your doctor about whether you should use this medicine.

The influenza virus A can develop resistance to the antiviral flu medicine amantadine. Although influenza viruses A and B have not developed resistance to zanamivir or oseltamivir at this time, resistance is possible with increased use of these medicines.

Of the three antiviral flu medicines, zanamivir and oseltamivir are the most expensive.

Complete the new medication information form (PDF) (What is a PDF document?) to help you understand this medication.



  1. Cooper NJ, et al. (2003). Effectiveness of neuraminidase inhibitors in treatment and prevention of influenza A and B: Systematic review and meta-analyses of randomised controlled trials. BMJ, 326(7401):1235.
  2. Shun-Shin M, et al. (2009). Neuraminidase inhibitors for treatment and prophylaxis of influenza in children: Systematic review and meta-analysis of randomised controlled trials. BMJ. Published online August 11, 2009 (doi: 10.1136/bmj.b3172).
  3. Treanor JT, et al. (2000). Efficacy and safety of the oral neuraminidase inhibitor oseltamivir in treating acute influenza. JAMA, 283(8): 1016–1024.
  4. Monto AS, et al. (2002). Zanamivir prophylaxis: An effective strategy for the prevention of influenza types A and B within households. Journal of Infectious Diseases, 186(11): 1582–1588.
  5. U.S. Food and Drug Administration (2006). Safety alert: Tamiflu (oseltamivir phosphate). FDA Safety Information and Adverse Event Reporting Program. Available online: http://www.fda.gov/Safety/MedWatch/Safetyinformation/SafetyAlertsforHumanMedicalProducts/ucm150758.htm.
  6. U.S. Food and Drug Administration (2008). Safety alert: Relenza (zanamivir). FDA Safety Information and Adverse Event Reporting Program. Available online: http://www.fda.gov/Safety/MedWatch/Safetyinformation/SafetyAlertsforHumanMedicalProducts/ucm094982.htm.


By Healthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD - Internal Medicine
Primary Medical Reviewer Brian D. O'Brien, MD - Internal Medicine
Specialist Medical Reviewer Christine Hahn, MD - Epidemiology
Last Revised September 17, 2010

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