Washington, Dec 23 : As Omicron variant spreads globally, the US Food and Drug Administration (FDA) has issued an emergency use authorisation (EUA) for the Pfizer antiviral pill to treat Covid-19 along with caution for people with certain pre-existing conditions -- heralding a new era for the future, over-the-counter treatment of the deadly respiratory disease that has killed millions. Pfizer's Paxlovid (nirmatrelvir tablets and ritonavir tablets, co-packaged for oral use) has been approved for the treatment of mild-to-moderate coronavirus disease (Covid-19) in adults and paediatric patients (12 years of age and older weighing at least 40 kg) with positive results of direct SARS-CoV-2 testing. The pill has also been approved for those at high risk for progression to severe Covid-19, including hospitalisation or death. Paxlovid is available by prescription only and should be initiated as soon as possible after diagnosis of Covid-19 and within five days of symptom onset, the FDA said in a statement late on Wednesday. "The authorisation introduces the first treatment for Covid-19 that is in the form of a pill that is taken orally -- a major step forward in the fight against this global pandemic," said Patrizia Cavazzoni, director of the FDA's Center for Drug Evaluation and Research. "This authorisation provides a new tool to combat Covid-19 at a crucial time in the pandemic as new variants emerge and promises to make antiviral treatment more accessible to patients who are at high risk for progression to severe Covid-19," Cavazzoni added. Paxlovid is not authorised for the pre-exposure or post-exposure prevention of Covid or for initiation of treatment in those requiring hospitalisation due to severe or critical Covid-19. The pill is also not a substitute for vaccination in individuals for whom Covid-19 vaccination and a booster dose are recommended. Paxlovid consists of nirmatrelvir, which inhibits a SARS-CoV-2 protein to stop the virus from replicating, and ritonavir, which slows down nirmatrelvir's breakdown to help it remain in the body for a longer period at higher concentrations. Paxlovid is administered as three tablets (two tablets of nirmatrelvir and one tablet of ritonavir) taken together orally twice daily for five days, for a total of 30 tablets. The pill is not authorised for use for longer than five consecutive days. The primary data supporting this EUA for Paxlovid are from EPIC-HR, a randomised, placebo-controlled clinical trial studying Paxlovid for the treatment of non-hospitalised symptomatic adults with a laboratory confirmed diagnosis of SARS-CoV-2 infection. Patients were adults 18 years of age and older with a prespecified risk factor for progression to severe disease or were 60 years and older regardless of prespecified chronic medical conditions. "Possible side-effects of Paxlovid include impaired sense of taste, diarrhoea, high blood pressure and muscle aches. Using Paxlovid at the same time as certain other drugs may result in potentially significant drug interactions," the FDA said. Using Paxlovid in people with uncontrolled or undiagnosed HIV-1 infection may lead to HIV-1 drug resistance. "Ritonavir may cause liver damage, so caution should be exercised when giving Paxlovid to patients with pre-existing liver diseases, liver enzyme abnormalities or liver inflammation," the FDA warned. /IANS