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Oseltamivir: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on Oct 20, 2022.

1. How it works

  • Oseltamivir is an antiviral agent that may be used to treat acute and uncomplicated infections due to influenza A or B viruses in people who have been symptomatic for no more than 48 hours.
  • The manufacturer suggests oseltamivir works by inhibiting viral neuraminidase, an enzyme that enables viruses to be released from their host cell. However, a Cochrane review suggests oseltamivir reduces symptoms of influenza by inhibiting the release of pro-inflammatory cytokines which reduces the immune response. It may also work centrally (through the brain) to lower temperature. The Cochrane reviewers did not find any evidence of influenza-virus-specific action.
  • Oseltamivir belongs to the class of medicines known as antivirals.

2. Upsides

  • Oseltamivir can be used to treat influenza A or B in adults and children aged two weeks or older who have been symptomatic for no more than 48 hours.
  • Oseltamivir may be used to reduce the risk of adults and children aged one year or older from becoming infected with influenza A or B viruses.
  • May be more effective at preventing the symptoms of influenza rather than treating them.
  • Available as a capsule and a powder for oral suspension.
  • Oseltamivir is a generic, and typically costs less than the branded version, Tamiflu.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • Nausea, vomiting, diarrhea, and headache are the main side effects. Rarely, allergic or skin reactions and breathing problems have been reported.
  • Oseltamivir has been associated with neuropsychiatric side effects (mainly confusion, delirium, and abnormal behavior that has resulted in injury; some cases were fatal). Most of these effects occurred in children and had an abrupt onset and a rapid resolution.
  • Oseltamivir does not take the place of early annual influenza vaccination.
  • There is always a risk that some influenza type A or B viruses may have developed resistance to oseltamivir and its antiviral effects. Doctors may need to consider local oseltamivir susceptibility patterns before deciding to use oseltamivir.
  • Rarely, anaphylaxis and serious skin reactions have been reported with oseltamivir.
  • Dosage may need adjusting in people with moderate-to-severe kidney disease. Avoid in people with end-stage kidney failure not undergoing dialysis. May not be suitable for some people with heart or lung disease, hereditary fructose intolerance, a weak immune system, or other serious health problems.
  • Some studies have raised doubt about the effectiveness of oseltamivir as a treatment for influenza. A Cochrane review found that oseltamivir shortened the duration of symptoms of influenza-like illness by less than a day. Oseltamivir is not effective against colds or other viral illnesses.
  • Oseltamivir will not prevent any secondary bacterial infections associated with the flu.
  • Oseltamivir may interact with some other medications including the live attenuated influenza vaccination and clopidogrel.
  • There is a lack of adequate or well-controlled studies regarding the use of oseltamivir during pregnancy; however, available data do not show a risk of birth defects in women exposed to oseltamivir at any trimester. Pregnant women are at higher risk of severe complications from influenza, which may lead to adverse pregnancy or fetal outcomes including maternal death, stillbirths, birth defects, preterm delivery, low birth weight, and small for gestational age. Animal studies have shown an increased risk of minor skeletal malformations at higher than normal human doses. Oseltamivir is excreted into breastmilk but the levels are so they are unlikely to affect a breastfed infant.

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

  • Oseltamivir is an antiviral medicine that is used for the treatment or prevention of the flu. When used to treat the flu, it is most effective if started within 48 hours of symptom onset.

5. Tips

  • Oseltamivir may be taken either with or without food. Taking it with food may reduce any gastrointestinal side effects such as nausea or vomiting.
  • Oseltamivir is usually taken twice daily when used for the treatment of influenza, or once daily when used to reduce the risk of catching influenza.
  • Oseltamivir is usually taken for five days when used for treatment and 10 days when used for prevention. However, it may be continued for 6 to 12 weeks in certain situations.
  • Oseltamivir should be started within 48 hours of symptom onset.
  • Stop taking oseltamivir and see your doctor urgently if you develop any signs of an allergic reaction to oseltamivir (such as facial swelling or a skin rash), or if the person taking oseltamivir develops any worrying or bizarre behaviors, or appears confused or delirious.
  • Oseltamivir only treats viruses, it will not treat bacterial infections. Do not use oseltamivir to treat any other infection, or give it to others to take.
  • You should not forgo your annual influenza vaccination just because oseltamivir is available. Continue to get a flu shot every year, based on your doctor's advice.
  • Tell your doctor if you are breastfeeding or pregnant because oseltamivir may not be suitable for you.

6. Response and effectiveness

  • Oseltamivir is rapidly absorbed from the gastrointestinal tract and converted by liver enzymes to its active form oseltamivir carboxylate (OC). OC is detected in the blood within 30 minutes of an oral dose. Peak concentrations of OC are reached within three to four hours.
  • Some studies have shown oseltamivir only reduces the duration of an influenza-like illness by about a day.

7. Interactions

Medicines that interact with oseltamivir may either decrease its effect, affect how long it works, increase side effects, or have less of an effect when taken with oseltamivir. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with oseltamivir include:

  • clopidogrel
  • hepatitis B medications, such as entecavir
  • live influenza vaccine (eg, FluMist Quadrivalent) given either two weeks before or 48 hours after oseltamivir administration. Note that inactivated flu vaccines can be administered at any time relative to oseltamivir use
  • methotrexate
  • pemetrexed, a chemotherapy agent
  • warfarin, an anticoagulant (blood thinner).

Alcohol should be avoided while taking oseltamivir because it weakens the immune system and may exacerbate the side effects of oseltamivir.

Note that this list is not all-inclusive and includes only common medications that may interact with oseltamivir. You should refer to the prescribing information for oseltamivir for a complete list of interactions.


Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use oseltamivir only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Copyright 1996-2023 Revision date: October 19, 2022.