Favipiravir is currently licensed in Japan to treat influenza.
It's the sixth medication to be entered into Oxford University's PRINCIPLE trial, but the first antiviral drug.
From today, Favipiravir will be investigated as part of the Platform Randomised trial of Interventions against COVID-19 In older people (PRINCIPLE) trial, which is the world's largest clinical trial of possible COVID-19 treatments for recovery at home and in other non-hospital settings.
The trial was set-up with the intention that drugs shown to have a clinical benefit could be rapidly introduced into routine NHS care.
Favipiravir works by inhibiting a viral enzyme called RNA polymerase, preventing viral replication within human cells. This viral enzyme is common to several viruses, including SARS-CoV-2, which causes COVID-19.
The drug has shown positive results against Covid in the lab and animal studies, with small pilot studies in humans 'demonstrating some benefit in reducing symptoms and the duration of illness.'
PRINCIPLE trial Co-lead Investigator, Chris Butler, a Professor of Primary Care at Oxford, said: 'Viruses need human cells to multiply and survive, and favipiravir blocks the complicated molecular dance that happens between a virus and our own cells when the virus is replicating.
'Until now, we have been testing medicines that have not typically been used to treat viral infections. This is the first drug we will be testing that was designed specifically to target viruses, so we are particularly excited to be including favipiravir in the PRINCIPLE trial to determine whether it could be used in the community as a COVID-19 treatment and prevent people from getting very sick.'
Launched in March 2020, PRINCIPLE has so far recruited more than 4,700 volunteers from across the UK.
It is recruiting participants who are most at risk of serious COVID-19 illness, either due to their age, symptoms, or an underlying health condition.
Co-Investigator Gail Hayward, an Oxfordshire GP and Associate Professor in the University of Oxford's Nuffield Department of Primary Care Health Sciences, said: 'Even with the successful vaccine rollout, it's the combination of vaccines, testing and treatments that will bring an end to this pandemic.
'There still isn't a safe and effective therapeutic for COVID-19 that we know for sure will prevent people needing to go into hospital for treatment. By joining the PRINCIPLE trial, people with coronavirus symptoms could play a vital role in helping to transform COVID-19 into an illness that can potentially be treated by your regular general practitioner.'
Following a screening questionnaire to confirm eligibility, participants enrolled in the study will be randomly assigned to receive an initial two doses of 1800mg favipiravir on day one, followed by 800mg twice a day for a further four days.
They will be followed-up for 28 days and will be compared with participants who have been assigned to receive the usual standard of NHS care only.
People aged 50 to 64 with certain underlying health conditions or shortness of breath from COVID-19, or aged over 65, are eligible to join the favipiravir arm of PRINCIPLE within the first 14 days of experiencing COVID-19 symptoms or receiving a positive test.
Other treatments under investigation by the Oxford team include budesonide, an inhaled steroid typically used for treating lung inflammation, and colchicine, an anti-inflammatory drug traditionally used for treating gout, which is being evaluated in adults aged over 18.
The trial has so far found that the antibiotics azithromycin and doxycycline are not generally effective as treatments during the early stages of COVID-19.
It's supported by the NIHR and a vast network of health and care professionals in care homes, pharmacies, NHS 111 Hubs, hospitals and more than 1000 GP practices across England, Wales, Scotland and Northern Ireland.
To find out more about how to join the study, CLICK HERE.