A trial investigating the drug hydroxychloroquine as a preventative treatment against Covid-19 may never find out if it's effective, say scientists involved.
Controversy around the drug - touted by President Trump and the subject of online misinformation - is stopping completion of the trial, they say.
It is ineffective in hospitalised patients, but investigators hope it might work if given earlier on.
Hospitals have pulled out of the trial.
'Intense politicisation and negative publicity'
The Oxford University-led trial is aiming to enrol 40,000 frontline workers around the world.
Investigators hope the large-scale, double-blind randomised study will show if early use of the treatment prevents the virus from getting worse.
"We know now that it doesn't work in treatment of hospitalised patients," says Prof Nick White, one of the study's investigators.
"But it's still is a medicine that may prove beneficial in preventing Covid-19."
The UK medicines regulatory body MHRA halted hydroxychloroquine trials, following a now-discredited paper in The Lancet claiming it caused harms.
Trials resumed in late June but the investigators says these concerns over safety, and the drug's politicisation, have made it difficult to get participants.
Prof White says hospital trusts have pulled out of the trial.
Why it's become controversial
Hydroxychloroquine has been used to treat malaria for years.
But there is currently no proof it works against coronavirus.
And the World Health Organization (WHO) has warned against misuse of the drug because of serious side-effects.
High-profile figures such as Mr Trump and Brazilian President Jair Bolsonaro have repeatedly promoted the drug - both taking it themselves.
And social media companies have removed viral online posts by doctors who reject the scientific consensus, praising the drug's effectiveness.
"I don't think there's been a more politicised and controversial medicine than hydroxychloroquine," says Prof White.
The UK Recovery trial dropped the drug after concluding it "does not save lives" of hospitalised patients.
There have yet to be results from large-scale studies on the drug's effectiveness as a prophylactic.
Healthcare workers have been disproportionately affected by Covid-19, and the Oxford scientists leading the trial hope the drug may help those on the frontline of the pandemic.