Pfizer Inc and Merck & Co Inc have developed experimental antiviral pills that have shown promising efficacy in trials of adults with Covid-19 who are at high risk of serious illness. Both drugs also are being studied to see if they can prevent infection in people exposed to the virus.

Here is an explanation of the differences in the two pills.

Which of the new pills works better?

Trial figures provided by the two companies suggest that Pfizer has the more effective pill, but they have not yet offered full data.

Pfizer said on Friday trial results showed that its pill reduced the chance of hospitalization or death by 89% in Covid-19 patients at risk for severe illness given the treatment within three days of the onset of symptoms and by 85% when given within five days of onset.

Merck on Oct. 1 said its pill lowered the chance of hospitalization or death by about 50% in patients at risk for severe illness given the treatment within five days of onset. It did not provide figures regarding patients getting the pill within three days of onset.

Pfizer's drug has the brand name Paxlovid. Merck's drug has the brand name Lavgevrio in Britain, where it has won regulatory approval.

Why are these drugs important?

While a number of vaccines are available worldwide to prevent infection including one made by Pfizer, there are limited treatment options for people infected with Covid-19.

Currently, Covid-19 patients who are not sick enough to be hospitalized but are at risk of serious illness can be treated with antibody drugs, though they have to be given intravenously at hospitals or infusion centers.

How do they work?

Both drugs are given for five days. Pfizer's regimen is three pills in the morning and three pills at night. Merck's drug is taken as four pills in the morning and four at night.

Pfizer's drug is part of a class known as protease inhibitors designed to block an enzyme that the coronavirus needs to multiply. Pfizer said that because the drug targets a part of the virus essential to replication, the pathogen cannot become resistant to the treatment.

Pfizer's drug is given in combination with ritonavir, an older antiviral that boosts the activity of protease inhibitors but can cause gastrointestinal side effects and interfere with other medications.

Merck's pill, developed with Ridgeback Biotherapeutics, is a nucleoside analogue with a mechanism of action that aims to introduce errors into the genetic code of the virus. Because the drug generates random mutations into the virus, it is difficult for the coronavirus to evolve and become resistant.

What do we know about safety?

Both companies have released only limited data on the treatments, but expressed confidence in their safety.

Pfizer said about 20% of patients who received either the pill or a placebo experienced adverse events, mostly mild. Serious side effects were reported by 1.7% of patients receiving the drug and 6.6% of placebo patients.

Merck said 12% of patients receiving its drug and 11% of placebo patients experienced drug-related adverse events.

Drugs in the same class as Merck's pill have been linked to birth defects in animal studies. Merck has said similar studies of its drug - for longer and at higher doses than used in humans - show that it does not cause birth defects or cancer.

What do we know about supplies?

Pfizer and Merck have said they are making efforts to expand global access to the drugs. Pfizer said it expects to produce more than 180,000 courses of its therapy by the end of this year, with production of at least 50 million planned for 2022. Merck has said it expects to produce 10 million courses of its drug by the end of this year, with at least 20 million set to be manufactured in 2022.

Which costs more?

The US government provides vaccines and treatments for COVID-19 for free to US residents. Countries around the world are negotiating prices with Pfizer and Merck.

President Joe Biden on Friday said the US government has secured millions of doses of Pfizer's treatment. Merck has a $1.2 billion contract to supply the United States with 1.7 million courses of its drug - or about $700 per course.

Britain has secured 250,000 courses of Pfizer's drug, but prices for the British contracts have not been made public.