Health Canada has approved Pfizer’s COVID-19 pill, Paxlovid, and the company says it has already shipped some to Canada for use. Dubbed a “game changer” in the battle against the coronavirus pandemic, it is the first oral medication approved for COVID-19 that can be taken by patients at home.

The agency authorized Paxlovid for adult patients with mild or moderate COVID-19 who are also at high risk of becoming more seriously ill, but it did not authorize it for use on teenagers or on patients who are already hospitalized because of COVID-19.

Paxlovid is a combination of two different drugs, to be taken together twice a day for five days.

Dr. Supriya Sharma, chief medical adviser with Health Canada, said authorization comes at a “crucial time in the pandemic as we’re faced with new variants.”

The drug could help alleviate pressure on health-care systems by decreasing the number of high-risk people who would require hospitalization, but health officials noted on Monday that supply would be an issue early on.

Dr. Theresa Tam, Canada’s chief medical officer, said Paxlovid’s impact likely won’t be seen right away.

“A lot of it depends on the initial supply and we all know that the supply isn’t going to be great at the start,” she said. “For the Omicron wave itself, it may contribute, but it won’t be a key contributor to the current wave. “So, it is very much another layer, another tool as we progress over the next month.”

Federal Health Minister Jean-Yves Duclos said Canada has already received its first shipment of 30,000 treatment courses of the Pfizer drug, with another 120,000 expected through March. He added that distribution to provinces and territories will begin “immediately.”

Procurement Minister Filomena Tassi says that obtaining Paxlovid will help keep Canada’s hospitalization figures from continuing to creep upwards. “I think the timing of this (authorization) is also very important,” she said. “The bottom line… it keeps people potentially out of the hospitals.”

The prescription medication is meant to be taken at home within five days of the start of symptoms, timing that could prove difficult as Canada deals with a limited capacity for PCR testing.

Tam said rapid tests could also be used to determine who might receive Paxlovid.

“You can try and get the PCR test but if it’s simply not available or not available fast enough, the rapid antigen test may be used,” she said. “That’s just a very practical approach.”

Tam said provinces and territories will determine how best to use limited supply of the drug, adding that deployment to jurisdictions will be based on a per-capita basis.

She said priority for Paxlovid will be given to those who are moderately to severely immunocompromised and don’t mount enough protection against COVID-19 with vaccines; people over the age of 80 whose vaccines are not up to date; and those 60 years and older living in rural or underserved communities including First Nation, Inuit, and Metis individuals whose vaccinations are not up to date.

Unvaccinated people also remain at higher risk for severe disease, but Duclos said they shouldn’t view Paxlovid as a substitute for vaccination. “A drug is a treatment,” he said. “It’s much better not to have to be treated and the best way not to have to be treated is to be vaccinated.”

The Pfizer pill uses a combination of two antiviral drugs to prevent the virus that causes COVID-19 from replicating once it has infected a patient. Sharma said clinical trials showed treatment with Paxlovid reduced the risk of hospitalization and death caused by COVID-19 by 89% when the medications were started within three days of the beginning of symptoms, and by 85% when started within five days.

Health Canada reported on its website Monday that it received the submission for Paxolovid from Pfizer on Dec. 1, 2021 and conducted an expedited review, adding that Pfizer submitted the data to Health Canada on a rolling basis throughout the review process, as new information became available to facilitate a decision as quickly as possible.

The agency noted that “while the benefits of Paxlovid were found to outweigh the risks, it can cause interactions with other medications for some patients. As such, patients should discuss the risks and benefits of treatment with their healthcare provider.”

Paxlovid was approved for use in the US on Dec. 22 and week later in the UK. Other countries, including Israel and South Korea have also sanctioned the drug.