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Antiviral drugs have sparked optimism in the fight against coronavirus, with President Donald Trump, New York Governor Andrew Cuomo and many more expressing hope for their success.

Some of the most-touted drugs, such as anti-malaria drug hydroxychloroquine, are already being prescribed off-label by some doctors in the United States for the coronavirus. New York State begins clinical trials using Tuesday.

The governments of China and South Korea also encourage the use of the drug in their official treatment guidelines. On Monday, India’s Council of Medical Research recommended its use in many cases. Hydroxychloroquine is cheap, generic and already widely available around the world as a malaria treatment.

But what is the evidence behind its use?

Multiple researchers have demonstrated that hydroxychloroquine is effective against the virus in test tubes in lab settings.

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But when it comes to studies on humans, the results are few. Doctors strongly warn against self-medicating with the drug, which can cause rare but serious side effects.

The most promising finding is a small study from doctors in Marseille, France. The authors treated 26 patients with hydroxychloroquine, and six patients were given both that and a drug called azithromycin.

President Trump tweeted about the study Saturday.

The authors did report dramatic recoveries.

“We have results which are significant to show that hydroxychloroquine is clearing the virus from the respiratory system,” lead author Didier Raoult said in a Youtube video discussing his findings.

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But researchers warn that the study has serious limitations. Out of the 26 patients originally given the treatment, the paper says that six were “lost in follow-up” and discontinued treatment early. The reasons are not encouraging: one patient being treated died on day three of the study, three went to critical care centers outside of the researchers’ hospital, one stopped due to getting nauseated as a side effect, and one got better and left the hospital early.

The strong positive effects the study reported are based on the other 20 patients who continued treatment.

The paper then compared those 20 patients to 16 patients at another hospital who did not get the drugs -- but those patients did not necessarily receive the same general care as the 20 patients receiving the treatment.

“Current available data are not strong,” French medical researcher Christian Perronne, who was not involved in the study, told Fox News, while adding that there is still reason to think it may work.

The rapid recovery of the 20 people who continued treatment offers a glimmer of hope. The French doctors have also released a treatment protocol that includes their recommended doses.

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Dr. Anthony Fauci, a member of President Trump's coronavirus task force, called the available evidence “anecdotal” on Friday.

“It was not done in a controlled clinical trial, so you really can’t make any definitive statement about it,” he said.

President Trump agreed.

“I agree with the doctor, what he said: It may work, it may not work. I feel good about it. That’s all it is.”

Some Americans are already using the drugs for coronavirus.

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Fox News reached one man in Manhattan whose doctor prescribed him the drugs recommended by the French researchers “off-label” after he had symptoms in line with COVID-19 and was unable to get quickly tested.

“I was in bad shape, I had a 103.5 degree fever, blood oxygen was around 94, heart rate of 118 beats per minute. ... I was in an unpleasant situation. I decided to take a chance,” he told Fox News. The man said he follows the medical literature closely.

“I did it knowing what I was getting into,” he said. “I personally found the side effects -- from digestive issues to feeling like I was caffeinated and drunk, to horrible taste in my mouth -- were unpleasant.”

Hydroxychloroquine can also in rare cases have more serious side effects, including causing psychiatric disorders and long-term vision impairment. The drug stays in a person for months after being taken, meaning it can’t be quickly undone if side effects are experienced. However, the drug is considered tolerable enough to be commonly used against serious conditions like malaria, lupus and arthritis.

The man reached by Fox said his side effects were tolerable and that his initial symptoms rapidly improved.

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“I feel glad I rolled the dice, but I would recommend any ordinary person talk carefully to their doctor first. Otherwise, ignore the drugs,” he said.

Medical researchers reached by Fox News urged caution.

“Everybody should follow Dr. Fauci: We just don’t know at this stage,” Xavier de Lamballerie, a researcher who published a paper calling for early release of Chinese data on the subject, told Fox News.

Chinese researchers stated in February that results from “more than 100 patients” show effectiveness, but they did not publish data backing that up.

The World Health Organization on Friday announced trials for hydroxychloroquine and the related drug chloroquine, but in a draft report on the drugs, a WHO working group noted that China has not shared data.

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“WHO is engaging with Chinese colleagues at the mission in Geneva and have received assurances of improved collaboration; however, no data has been shared regarding the chloroquine studies,” the report says.

At the same time, the World Health Organization draft mentioned reviewing an unspecified small Chinese study that found no benefit to the drug.

“[A] recently published [randomized controlled trial] study in China with hydroxychloroquine involving 30 COVID-19 patients with mild to moderate symptoms shows no significant reduction,” the WHO report reads.

That small study may be an outlier; in its latest national guidelines, China still recommends its doctors use the drug.

Better controlled trials are now ramping up all over the world to see if the anecdotes turn into conclusive evidence.

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New York Governor Andrew Cuomo announced on Sunday that the State has acquired 70,000 doses of hydroxychloroquine. “I've spoken with a number of health officials and there is a good basis ... I agree with the president on that and ... we're going to start Tuesday,” he said.

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If it doesn’t work, several other drugs are rapidly being researched as well. That includes remdesivir, an anti-viral originally designed to fight Ebola. Researchers are also trying to create antibodies specifically designed for the coronavirus, and a vaccine is under development.

Maxim Lott is Executive Producer of Stossel TV and creator of ElectionBettingOdds.com. He can be reached on Twitter at @MaximLott