University of Oxford scientists are trialling giving Ivermectin to people with Covid symptoms to see if it can keep them out of hospital.
The Principle study will compare those given the drug to patients receiving the usual NHS care.
The drug has become controversial after being promoted for use across Latin America and in South Africa, despite being so far unproven.
Previous studies of Ivermectin have generally been small or low quality.
Most commonly used to treat parasitic infections such as river blindness, spread by flies, Ivermectin has also been shown to kill viruses in petri dishes in the lab - although, at much higher doses than would usually be prescribed to people.
Dr Aurora Baluja, an anaesthesiologist and critical care doctor, said Ivermectin was often being given in parts of the world where there are high incidences of parasitic infections.
Covid patients who are also fighting a parasitic disease at the same time would be likely to fare worse and that might explains some of its seemingly positive effect.
Though there have been some early "promising" results from small and observational studies, Principle joint chief investigator Prof Richard Hobbs said it would be "premature" to recommend Ivermectin for Covid.
Observational studies look at people already taking the drug, rather than giving it to a group representative of the population.
So they fail to account for differences in the types of people who might choose that treatment, and other factors that might have been influencing the spread of the virus at the time.
