The Covid-19 treatment from Pfizer Inc. has become the leading pandemic pill prescribed in the U.S., as supplies have improved and its availability at pharmacies widened.

Pfizer’s antiviral drug, called Paxlovid, totaled more than 412,000 prescriptions through May 6, compared with about 110,000 prescriptions of molnupiravir, an antiviral from Merck & Co. and Ridgeback Biotherapeutics LP, according to drug-data firm Iqvia Holdings Inc.

The Biden administration is preparing to announce plans this week to make Pfizer's COVID-19 pill available at almost any pharmacy in the U.S. after early shortages.

Why it matters: The drug, Paxlovid, was called a potential "game-changer" for its ability to treat infections at home and keep individuals at risk of severe illness out of the hospital.

But availability has been spotty, making access difficult as COVID cases begin to rise from Omicron again.

While strongly recommending the use of Paxlovid for high-risk Covid-19 patients who develop mild or moderate symptoms, the World Health Organization on Friday warned that without increasing testing access and the reach of generic production and consumption, the lifesaving medicine is likely to remain inaccessible in much of the Global South—replicating the injustice of vaccine apartheid.

High-risk COVID-19 patients now have new treatments they can take at home to stay out of the hospital — if doctors get the pills to them fast enough.

Health systems around the country are rushing out same-day prescription deliveries. Some clinics have started testing and treating patients in one visit, an initiative that President Joe Biden’s administration recently touted.

Some pharmacies are getting paid as little as $1 to dispense the COVID antiviral pills made by Pfizer and Merck.

Why it matters: Most pharmacies want to offer the drugs, especially the one from Pfizer that drastically cuts the odds of hospitalization and death. But low payments could hurt Americans' ability to access the pills in some areas if pharmacies decide they can't afford to stock them.

How it works: Pharmacies are paid an "ingredient cost" and a "dispensing fee" for drugs.

Pfizer’s antiviral oral drug developed to treat COVID-19 can cause severe or life-threatening reactions when used with common medications including some anticoagulants, antidepressants, and cholesterol-lowering statins, according to a Food and Drug Administration (FDA) fact sheet (pdf).

Paxlovid is the first oral medication of its kind authorized by the FDA to treat COVID-19, with the aim of reducing the need for hospitalization before patients become too ill from the infection.

Experts warn that despite the first antiviral pills for COVID-19 promising protection for those at risk of severe disease, Pfizer's or Merck's new medications may not be safe for everyone, NBC News reported over the weekend.

One of the two drugs in Pfizer's Paxlovid antiviral cocktail could cause severe or life-threatening interactions with commonly used medications, including statins, blood thinners and some antidepressants, and the FDA does not recommend Paxlovid for those with severe kidney or liver disease.

As the omicron surge pummels a pandemic-weary nation, the first antiviral pills for Covid-19 promise desperately needed protection for people at risk of severe disease. However, many people prescribed Pfizer’s or Merck’s new medications will require careful monitoring by doctors and pharmacists, and the antivirals may not be safe for everyone, experts caution.

The Food and Drug Administration (FDA) recently authorized two antiviral pills, one from Pfizer and one from Merck, making them the first at-home treatment for COVID-19 that has been touted as a game changer in the fight against the pandemic.

However, experts told NBC News that the pills will require careful monitoring by doctors and pharmacists.