Limited global stocks of two anti-malarial drugs could wreck plans to use the medicines, currently in clinical trials, to treat COVID-19, doctors cautioned on Thursday.

Around the world, countries are expanding access to chloroquine (CQ) and hydroxychloroquine (HCQ), which are used to treat malaria and are known to have anti-viral properties.

The medicines have shown early promise against the COVID-19 illness in studies in France and China. CQ, which is the less toxic of the two, is also used as an anti-inflammatory to treat conditions like rheumatoid arthritis and lupus, purposes it is primarily known for outside the tropics.

Writing in the Annals of Rheumatic Diseases, doctors in Italy -- the country hardest hit by COVID-19 -- said that limited supply could scupper any widespread attempt to use the two drugs against the virus.

They also cautioned that it was vital that patients already taking chloroquine for other illnesses should not be deprived of their supply.

"In some European countries the availability of HCQ and CQ in the pharmacies (outside the hospitals) is already scarce," Francesca Romana Spinelli, assistant professor at the Sapienza University of Rome and letter author, told AFP. "This is an emerging problem for many patients already treated with CQ/HCQ for their autoimmune rheumatic disease."

Both medicines are known to have anti-viral properties and have shown some encouraging results in trials against COVID-19. But they have a number of potentially serious side effects, and there are fears that treating COVID-19 patients, many of whom are on medication for underlying conditions, could court disaster.

On Wednesday the European Medicine Agency warned that CQ and HCQ should only be used on COVID-19 patients in clinical trials or in case of a "national emergency".

Iain McInnes, president of the European League Against Rheumatism (EULAR), said that more research was needed before governments could consider rolling out the two drugs against the virus. He said he was concerned "that the diversion of drug supplies away from people with rheumatic and musculoskeletal diseases may compromise the health of this important and sizable group of patients in Europe and beyond."

In the letter, Italian doctors said using CQ and HCQ as widespread COVID-19 treatments would raise ethical concerns, given their known side effects. "If mass prophylaxis was accepted as an option worldwide, this would raise the question of whether there is enough supply of chloroquine and hydroxychloroquine to support this approach," they added.