Health Canada has approved Pfizer's antiviral treatment for COVID-19 and the drug maker says some of it has already been delivered for use.

The authorization posted to the Health Canada website Monday morning says the treatment can be used for adult patients with mild or moderate COVID-19 who are also at high risk of becoming more seriously ill.

The drug review team at Health Canada says the risk-benefit analysis was "favourable" for adults in this category.

Chief medical adviser Dr. Supriya Sharma says this is the first oral medication approved for COVID-19 that can be taken by patients at home. She said until now all authorized treatments were administered intravenously or by injection in a hospital.

Paxlovid is a combination of two different drugs, that are to be given within five days of infection. Together the drugs showed to be almost 90 per cent effective at preventing hospitalization or death among higher risk patients with COVID-19.

Canadian doctors have been anxious to get their hands on the medication as the Omicron wave is spreading so quickly. Canada bought one million courses of the treatment for delivery in 2022.

Pfizer spokeswoman Christina Antoniou told The Canadian Press the company "is ready to start immediate delivery in Canada."

"There is already some supply in Canada, which will begin to be distributed this week," she said.

Chief public health officer Dr. Theresa Tam says because of supply constraints the Public Health Agency of Canada is asking provincial and territorial governments to prioritize the highest-risk patients for the medications.

Severely immune-compromised patients, people over 80 who don't have all their vaccines, and people over 60 living in remote and rural locations, long-term care homes or First Nations top the list, Tam said.

Sharma said the treatment is two different tablets, taken together twice daily, for five days.

Supply issues however are making the drug difficult to find in the United States, where it was approved Dec. 22. The company plans to make 30 million courses of the treatment by the end of June, and 120 million by the end of the year. The U.S. is to get 10 million doses by June and another 10 million by September.

The United States Food and Drug Administration authorized Paxlovid for patients as young as 12 years old but Health Canada says the company did not submit any safety or efficacy data for that age group so it can't be authorized for people younger than 18 at this time.

Health Canada also says the treatment isn't to be used on patients already in hospital with severe or critical COVID-19 or as a prevention treatment before or after someone is exposed to the virus. It is also not to be given to a patient for more than five days.

It warns there are some potentially severe drug interactions between Paxlovid and other medications prescribed for ailments including prostate cancer, heart problems and narcotics including fentanyl.