The drug is not approved for treating Covid-19 but the president has regularly championed it despite the lack of scientific evidence behind its usage.
Hydroxychloroquine can have significant side effects and is only permitted to be used within a hospital setting.
After Mr Trump declared he's using the drug, the UK government advised against it.
Boris Johnson's spokesman said: “It’s not something which our own medical experts are recommending.”
But what do we know about the drug? Here are some answers to a few key questions:
What is hydroxychloroquine?
Hydroxychloroquine is a prescription drug used for acute malaria and certain types of arthritis.
It can reduce inflammation, pain, and swelling – and is widely used to treat rheumatic diseases.
It is a derivative of chloroquine, which is also used to treat malaria.
Has it been approved to treat Covid-19?
The UK Government has said that chloroquine and hydroxychloroquine are not licensed to treat Covid-19-related symptoms or prevent infection.
It said the drugs should not be used outside ongoing clinical trials which have reached no conclusions over the safety and effectiveness of the medicine on coronavirus.
What are the potential side effects?
Common side effects include headaches and dizziness, in addition to nausea, vomiting and stomach pain.
Additionally, it has been found to cause heart issues.
Dr Stephen Griffin, associate professor in the School of Medicine, University of Leeds, said: “Hydroxychloroquine is not licensed for the treatment or prevention of Covid-19 by the FDA (US Food and Drug Administration), or any other agency.
“In fact, the weight of evidence from most recent patient trials shows it to be ineffective, with the potential for adverse side effects including those affecting the heart.”
The FDA did issue an emergency use authorisation to allow the drug to be provided to certain hospitalised patients, but has since warned against the drug’s use outside clinical trials or hospital settings due to the risk of heart rhythm problems.
White House physician Sean Conley says the potential rewards outweigh the risks
"In consultation with our inter-agency partners and subject matter experts around the country, I continue to monitor the myriad studies investigating potential Covid-19 therapies, and I anticipate employing the same shared medical decision making based on the evidence at hand in the future," Conley wrote.
How have scientists reacted to Trump’s comments?
Dr Griffin said the president’s remarks were “a staggering, irresponsible act that could very well also amount to self-harm”.
He warned that hydroxychloroquine is prescribed and monitored carefully owing to potential side effects, and as such people following Mr Trump’s example could “endanger themselves”.
Meanwhile, former Government chief scientific adviser Sir David King, said of Mr Trump: “Every word he says should be ignored in terms of advice.”
He added: “I’m sorry but this is not the pronouncements of a person who is listening to the scientists. He is making it up as he goes along.”
But why is hydroxychloroquine being considered as a Covid-19 treatment?
According to the authors of a recent French study, attention turned to the drug after a laboratory study reported its potential.
One small uncontrolled study in France on the use of hydroxychloroquine in combination with another drug, azithromycin, found some reduction in patients’ viral load, while another small study found no such benefits.
Chinese researchers recently wrote that hydroxychloroquine is one of a number of drugs being look at for Covid-19 treatment based on promising lab results and therapeutic experiences with other coronavirus diseases.
But they emphasised that this has not been translated through trials into clinical benefits for patients with Covid-19.
What has recent research shown?
Evidence from two recent studies does not support the use of hydroxychloroquine to treat patients with Covid-19.
A study from France involving 181 patients found that it did not help significantly reduce admission to intensive care or death in patients hospitalised with pneumonia due Covid-19.
A randomised clinical trial carried out in China with 150 patients concluded that hospitalised patients with mild to moderate persistent Covid-19 who received hydroxychloroquine did not clear the virus more quickly than those receiving standard care.
What risks are there around hydroxychloroquine?
Dr Ayfer Ali, assistant professor of strategy at Warwick Business School and an expert in drug repurposing, said that without a vaccine using existing drugs could be the “only hope” of treating Covid-19.
But he warned this was not without challenges.
Dr Ali said: “A drug may be less safe for a different disease or in sicker patients.
“For example, preliminary data from the US indicates that patients with Covid-19 who took hydroxychloroquine were more likely to die than patients who did not take the drug.”
What other concerns are there?
Italian doctors have highlighted that ethics and the availability of drugs needs to be considered as part of any attempt of the widespread use of anti-malarials in the fight against coronavirus.
Concerns have been raised over whether there would be enough supply of chloroquine and hydroxychloroquine should they be used to treat coronavirus.
The European League Against Rheumatism (EULAR) has also said consideration must be given to the potential implications for people who already use the drugs for rheumatic diseases.
Coronavirus: Everything you need to know: