Baloxavir (Xofluza): A new antiviral drug for the flu

A new drug for the treatment of influenza was approved by the US Food and Drug Administration (FDA) in October 2018, just in time for the 2018–19 flu season. Baloxavir marboxil (Xofluza) works against the two types of influenza virus that cause disease in humans, influenza A and B.

This new drug works differently than currently available drugs, including oseltamivir (Tamiflu), zanamivir (Relenza), and peramivir (Rapivab). These older drugs inhibit the virus by blocking a viral enzyme called neuraminidase. In contrast, baloxavir inhibits a subunit of the viral polymerase, the enzyme responsible for influenza virus replication. Put more simply, baloxavir interferes with the ability of the flu virus to multiply, while the older drugs interfere with the ability of the flu virus to spread within the body.

This different mechanism of action means baloxavir may be effective against strains of influenza A that are resistant to oseltamivir, the drug used most widely for treatment of influenza. Baloxavir is also active against strains of avian influenza (“bird flu”).

Baloxavir reduces duration of flu-like symptoms

Baloxavir marboxil is taken by mouth. In the gastrointestinal tract, the marboxil component gives way to reveal the active drug, baloxavir acid. A phase 3 randomized, double-blind trial called CAPSTONE-1, published in the New England Journal of Medicine, compared baloxavir to oseltamivir and to placebo for the treatment of influenza-like illness. In this multicenter study (supported by the manufacturer of baloxavir), otherwise healthy patients, ages 20 to 64, received either a single dose of baloxavir (40 or 80 milligrams, depending on the weight of the patient); a twice-daily 75-mg dose of oseltamivir for five days; or placebo. In addition, children and adolescents ages 12 to 19 were randomized to receive either baloxavir or placebo.

CAPSTONE-1 showed that baloxavir reduced the duration of flu-like symptoms by about one day, from an average of 80.2 hours to 53.7 hours. Of note, patients who started baloxavir within 24 hours of symptom onset had a greater benefit from the drug compared to those who started later. Baloxavir rapidly reduced the concentration of influenza virus in respiratory secretions, and did so more rapidly than oseltamivir (one day versus three days, respectively). Nevertheless, both baloxavir and oseltamivir were similarly effective in shortening the duration of flu-like symptoms. Baloxavir was generally well tolerated.

For now, generic oseltamivir is still a good option

Given the results of CAPSTONE-1, when should baloxavir be used, and should it be used in place of oseltamivir? Baloxavir is approved for the treatment of influenza in patients 12 and older who have been symptomatic for no more than 48 hours. The main advantage of baloxavir is that a single oral dose is effective, whereas oseltamivir needs to be taken twice daily for five days. However, both drugs reduce the duration of flu-like symptoms. Oseltamivir is now available generically, and may be less expensive than baloxavir. Therefore, unless a doctor suspects that someone is unable or unlikely to complete a five-day course of medication, oseltamivir remains an appropriate choice for the majority of people with influenza, pending additional data or prescribing guidelines.

Related Information: Cold and Flu


  1. Hope

    My 18 year old daughter got the flu this week and WOW this was amazing. Unlike the tamaflu… she did not have vomiting and nausea. I am AMAZED at how well this worked and how quickly she started feeling better.

  2. Michael Glenn

    I had been ill with Influenza Type A for close to 48 hours when I was prescribed Xofluza. My temperature fell from 103.5. to normal in about 18 hours and my respiratory distress decreased considerably. I’ve remained fatigued, however, and have yet to recover my appetite 48 hours on. Also, I’ve had diarrhea since taking the medicine. I had read that diarrhea is a side effect of Xofluza. To be sure, the fever was worse than the current stomach distress, but I wonder how long it will take for my digestive system to recover.

  3. F. Lin

    In the past, my doctor prescribed Relenza and Tamiflu for different bouts with the flu. Both worked well but this year, the flu caused me to be bedridden for 40 hours. I was too weak to pick up a phone and was finally dragged by my wife to the same doctor as before. He gave me Xofluza and told me that it is a single pill dose and that it is shown to provide relief faster than the previous drugs without having to take multiple pills. At the time I had 102 degree temperature and could hardly walk unassisted with my body aching to the point where I couldn’t close my hand with any amount of force. I asked him if it was safe and he told me that though it was approved only a few months ago by the FDA, it was used in Japan for 2 years already and I trusted him. He told me he is prescribing to me what he would do for himself. The result is I felt relief within 4 hours and by the morning 8 hours later, I was still a little wobbly but I was able to work on my van for several hours and change batteries. I am writing this 24 hours after taking the Xofluza and I feel well enough to the point where I am the same as before I got sick (not including the sore rib, stomach, and muscles from the fierce coughing from the past 48 hours).

  4. C. Parks

    I just had the flue and took Xofluza. The next day I was 90% better, body temp down to 99.0 from 102 the day before. You are not cured in 24 hours, but your are close. I did have the flu vaccine, I haven’t missed one in 20 years. The only reason I knew Xofluza existed was that my daughter took it the week before 2 states away.
    My doctor tried to talk me into Tamiflu, but I asked for Xofluza. It wound up costing me $88.00 with a coupon and insurance at

  5. Reynaldo Cunanan

    I had a flu for 48 hours and my doctor prescribed Tamiflu for 5 days. I showed her the new drug Xofluza approved by the FDA, but she still insist on the former. She said the medical community still not ready to embrace the new drug. Is this true?

  6. Deepak Sharma

    Baloxavir marboxil is an antiviral medication used to treat the flu (influenza) if your symptoms started less than 48 hours ago. It helps make the symptoms (such as stuffy nose, cough, sore throat, fever/chills, aches, tiredness) less severe and may shorten the recovery time by 1 to 2 days. Baloxavir marboxil works by stopping the flu virus from growing. It will not treat other kinds of infection besides the flu virus. Baloxavir marboxil is not a substitute for the flu vaccine.

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