Nov 23 (Reuters) - The European Union"s drug regulator said on Tuesday it has started reviewing U.S. drugmaker Merck & Co Inc"s (MRK.N) experimental COVID-19 antiviral pill for adults following an application and could issue an opinion "within weeks." The European Medicines Agency (EMA) has already evaluated a substantial portion of the data during a rolling review that began in October. Merck"s pill Lagevrio has shown it can halve the chances of dying or being hospitalized for those most at risk of developing severe COVID-19 when given early in the illness. It was developed with Ridgeback Biotherapeutics and won approval from Britain earlier this month. read moreWhile vaccines are central to the fight against the pandemic, regulators are looking at therapies and the EMA last week began a review of Pfizer Inc"s (PFE.N) rival tablet. U.S. advisers will meet on Nov. 30 to review the safety and efficacy data on Merck"s drug and vote on whether it should be authorized.