The drugmaker said its pill showed a 30% reduction in hospitalisations and deaths, based on data from over 1,400 patients. In October, its data showed a roughly 50% efficacy, based on data from 775 patients.
Merck's shares fell 2.8% to $80 in premarket trading, amid a fall in the broader markets.
Potential authorisations of Merck's drug molnupiravir and a rival pill being developed by Pfizer Inc are hoped to be game-changers as they can be taken as early at-home treatments to help prevent hospitalisations and deaths.
Data from Pfizer's pill, following analysis of a study involving 1,200 participants, showed an 89% reduction risk of coronavirus-related hospitalisations or deaths compared with a placebo.
Merck filed for a US authorisation of molnupiravir on Oct. 11, following the interim data.
The company said on Friday data on molnupiravir, developed with Ridgeback Biotherapeutics, had been submitted to the US Food and Drug Administration ahead of a meeting of its expert advisers on Tuesday.
The molnupiravir arm of the study had a hospitalisation and death rate of 6.8%, according to the updated data. The placebo group had a hospitalisation and death rate of 9.7%.
One patient in the molnupiravir arm died, versus nine in the placebo group.
A planned interim analysis of the data last month showed 7.3% of those given molnupiravir twice a day for five days were hospitalised and none had died by 29 days after the treatment, compared with a rate of 14.1% for patients on placebo.
"We continue to believe that governments will continue to order both Merck's molnupiravir and Pfizer's Paxlovid initially given rising prevalence and concerns over resistance," Citi analyst Andrew Baum said.