Drug Information

Oseltamivir or Zanamivir

Oseltamivir or Zanamivir


Generic Name Brand Name
oseltamivir Tamiflu
zanamivir Relenza

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, because that is where the influenza (flu)  virus multiplies. It is inhaled twice a day for 5 days.

How It Works

Oseltamivir and zanamivir 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

Oseltamivir and zanamivir 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.
  • Oseltamivir is used to treat flu in people age 1 and older.
  • Zanamivir is used for people age 7 and 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

Oseltamivir and zanamivir may help prevent or treat the flu. 1, 2 Using one of these medicines typically shortens the course of influenza A or B by about 1 day. 1, 3

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Zanamivir may prevent flu among household members when a member of the family is infected. 4

Oseltamivir and zanamivir 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

All medicines have side effects. But many people don’t feel the side effects, or they are able to deal with them. Ask your pharmacist about the side effects of each medicine you take. Side effects are also listed in the information that comes with your medicine.

Here are some important things to think about:

  • Usually the benefits of the medicine are more important than any minor side effects.
  • Side effects may go away after you take the medicine for a while.
  • If side effects still bother you and you wonder if you should keep taking the medicine, call your doctor. He or she may be able to lower your dose or change your medicine. Do not suddenly quit taking your medicine unless your doctor tells you to.

Call 911 or other emergency services right away if you have:

  • Trouble breathing.
  • Swelling of your face, lips, tongue, or throat.
  • Confusion or delirium .
  • A change in behaviour, or thoughts of harming yourself or others.
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Call your doctor if you have:

  • Hives.

Common side effects of these medicines include:

  • Diarrhea.
  • Nausea and vomiting.
  • Swelling of the sinuses (sinusitis).

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

What To Think About

Oseltamivir and zanamivir 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.

Although influenza viruses A and B have not yet developed resistance to oseltamivir and zanamivir, resistance  is possible with increased use of these medicines.

Taking medicine

Medicine is one of the many tools your doctor has to treat a health problem. Taking medicine as your doctor suggests will improve your health and may prevent future problems. If you don’t take your medicines properly, you may be putting your health (and perhaps your life) at risk.

There are many reasons why people have trouble taking their medicine. But in most cases, there is something you can do. For suggestions on how to work around common problems, see the topic Taking Medicines as Prescribed.

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Advice for women

If you are pregnant, breast-feeding, or planning to get pregnant, do not use any medicines unless your doctor tells you to. Some medicines can harm your baby. This includes prescription and over-the-counter medicines, vitamins, herbs, and supplements. And make sure that all your doctors know that you are pregnant, breast-feeding, or planning to get pregnant.


Follow-up care is a key part of your treatment and safety. Be sure to make and go to all appointments, and call your doctor if you are having problems. It’s also a good idea to know your test results and keep a list of the medicines you take.




  1. 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).
  2. Hsu J, et al. (2012). Antivirals for treatment of influenza: A systematic review and meta-analysis of observational studies. Annals of Internal Medicine, 156(7): 512–524.
  3. Jefferson T, et al. (2012). Neuraminidase inhibitors for preventing and treating influenza in healthy adults and children. Cochrane Database of Systematic Reviews (1).
  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.