The world's largest study into the use of a controversial flu drug in the community has just reached 2,000 participants, researchers have said.
Scientists at the University of Oxford and other European institutions have been on a drive to recruit people to test the effectiveness of oseltamivir - known as Tamiflu - in primary care since 2015.
This week, the ALIC4E trial reached 2,000 participants, making it the largest clinical trial into an antiviral drug, staff at the university's Nuffield Department of Primary Care Health Sciences said.
Patients and GPs from 16 countries are now involved in the study, including Ireland, Belgium, the Czech Republic, Denmark, France, Greece and Hungary.
The milestone comes during one of the worst flu epidemics in the UK of recent years, with NHS hospitals ordered to cancel all non-urgent surgery until February to tackle the winter crisis.
Tamiflu is a type of drug known as a neuraminidase inhibitor - these are stockpiled and used across the globe to treat seasonal and pandemic flu outbreaks.
But experts say there is not enough evidence to support their effectiveness, as - despite being widely used during the swine flu pandemic - no trial was done on their clinical and cost effectiveness.
Professor Chris Butler, one of the lead investigators, said: "We are celebrating an incredible international achievement of having recruited over 2,000 patients into the critically important ALIC4E study.
"Especially when there seems to be a particularly widespread flu outbreak, it's a real shame that we don't confidently know which people with symptoms of the flu should be prescribed antiviral drugs, and the cost-effectiveness of this treatment in terms of helping people return to their usual activities."
The trial is an initiative of the Platform for European Preparedness Against (Re-)emerging Epidemics consortium, known as PREPARE, and is funded by the European Commission.
PREPARE was set up to help research organisations respond quickly to pandemics through clinical studies to inform the response of policymakers.