From the Lancet:
https://www.thelancet.com/journals/lanrhe/article/PIIS2665-9913%2820%2930089-8/fulltext
“Whether hydroxychloroquine works in vivo is not proven for any virus, and in fact in randomised controlled trials against a number of viruses, including influenza, it doesn't work at all,” says Douglas Richman, a virologist and infectious disease physician at the University of California, San Diego. “It's my personal prejudice that this is also going to be the case with coronavirus.”
Hydroxychloroquine has been studied as a possible antiviral for approximately the past 40 years, says Richman. The mechanism of action is not entirely clear, but it is known to decrease the acidity in endosomes, which might prevent the endosome from releasing the virus into the cytoplasm.
Hydroxychloroquine has shown activity in vitro against many viruses, including influenza and coronaviruses, but that has largely failed to translate into success in either animals or humans. In 2005, the drug showed in vitro activity against SARS-CoV, which is closely related to the current pandemic virus, but it failed to decrease viral load in mice, and clinical interest drifted away, says Christopher Tignanelli, a surgeon at the University of Minnesota in Minneapolis, who is involved in clinical trials of COVID-19 treatments.
“There is not a huge amount of pre-clinical data for this drug,” says Tignanelli. “It's mostly test-tube and anecdote.”
Despite the absence of strong evidence, some people are already attempting to self-medicate with the drug, with disastrous consequences. Hydroxychloroquine can have dangerous side-effects if the dose is not carefully controlled, and cases of chloroquine poisoning have been reported in Nigeria and the USA.