Two more also show promise.
Lopinavir-Ritonavir
There are three ways to fight the virus: antivirals, anti-inflammatories or antibodies.
Antivirals prevent the virus building to sufficient levels, while anti-inflammatories stop the immune system from overreacting and mounting a lethal inflammatory response.
Antibodies are produced by the immune system to defend against an invader and are boosted by a vaccine. But infusions of antibodies from recovered patients can also be used as a treatment.
Lopinavir-Ritonavir is a combination of two antiviral drugs used to fight HIV and work by inhibiting the enzymes required for the virus to replicate.
Trials have had mixed results. A study published in April of 86 patients at Guangzhou hospital in China found the combination made no difference to the death rate or time to recovery.
But earlier tests on 199 patients at Jin Yin-Tan Hospital showed those given the drug appeared to improve faster. Some experts believe it may help to give the treatment earlier.
When the Chinese results were released, Ian Jones, professor of virology at the University of Reading, said: "Think bull in a china shop. If the bull is stopped as it leaves the shop the damage is already done but if it is stopped soon after entry, the damage is much reduced."
Early in the outbreak, doctors in Thailand reported success in treating patients using a combination of HIV drugs alongside oseltamivir, a drug sold under the brand name Tamiflu to treat influenza.
Azithromycin
This common antibiotic is widely used to treat chest, sinus, throat and skin infections, traveller's diarrhoea and sexually transmitted diseases.
The drug is so important it is on the World Health Organisation's list of essential medicines.
Azithromycin prevents bacteria from growing by interfering with their ability to produce proteins and is often used as an alternative for patients allergic to penicillin. But it can also stop inflammation and viral replication and is known to prevent the production of cytokines, which are important messengers for the immune system.
Many patients who die from the virus suffer a so-called "cytokine storm" in which the body reacts so aggressively to the invader that it causes dangerous blood-clotting (thrombosis) in the lungs, which prevents breathing.
Immune cells go beyond infected parts of the body, damaging healthy tissues and stopping organs getting enough blood.
A rapid review by Oxford University found mixed results.
A trial found those taking it alongside hydroxy-chloroquine cleared the virus faster.
Tocilizumab
The drug suppresses the immune system, and again prevents the cytokine storm which can prove fatal. It is used to treat immune system disorders such as rheumatoid arthritis.
In April, trials on 129 patients in France showed the drug could significantly decrease the number of deaths or life support interventions.
Around half the trial's participants received two tocilizumab injections and standard antibiotics therapy, while the rest had standard treatment.
The findings, yet to be published, were said to indicate clear "clinical benefit" and results from the Oxford trial were likely to be released soon.
Convalescent blood plasma
People who recover from coronavirus usually carry powerful antibodies that can fight it off. Scientists are hopeful that transfusing them into sick people could boost the immune system.
The idea is not new. In the Spanish flu pandemic of 1918 doctors used blood serum from recovered patients successfully to treat the sick.
More recently, plasma transfusion was used experimentally to treat small numbers of people during the Sars outbreak of 2002 and 2003. British hospital patients who have recovered from Covid-19 are being asked to donate their blood, and from this week, anyone who tests positive will also be asked to give blood.
In April, a trial by Shanghai Jiao Tong University School of Medicine in China reported that 10 severely ill patients made a speedy recovery after being given blood plasma.
The US Food and Drug Administration has already approved blood plasma therapy as an experimental treatment in clinical trials, and for critical patients with no other option.
British scientists are hoping to publish results from blood plasma trials soon.
Remdesivir
The broad-spectrum antiviral is undergoing global clinical trials and is being given to seriously ill coronavirus patients in Britain to help speed their recovery.
The drug works by disrupting the genetic code of a virus to prevent it replicating. Many viruses reproduce in similar ways so it is hoped the drug will be useful for several infections and new diseases that may emerge.
Early data showed it may shorten the time to recovery by about four days and the Medicines and Healthcare products Regulatory Agency has cleared the treatment for use in the Early Access to Medicines Scheme.
Remdesivir was developed by Gilead pharmaceuticals a decade ago to cure "Disease X" amid fears that a global pandemic was looming.
More than a dozen centres across the NHS have been trialling it since April and results have been positive.
Hydroxychloroquine
Antimalarial hydroxychloroquine trials were recently stopped by Oxford University after showing no benefit - and possible harms.
It was hoped the drug would stop the virus entering cells. Studies in France and China reported patients who did not have severe symptoms recovered faster.
It was praised by Donald Trump who said he took it himself to ward off the virus.
But the Recovery trial's results released on June 5 showed no clinical benefit for patients with Covid-19. However, trials continue to see if giving the drug early could be beneficial.
Under the Principle (Platform Randomised trial of Interventions against Covid-19 in older people) trial by Oxford University, older and vulnerable people with the disease will be given the medication to see if it can prevent the need for hospital treatment.
Chris Butler, the chief investigator and professor of Primary Care in the Nuffield Department of Primary Care Health Sciences, said: "As soon as we find that any one of the drugs in our trial is making a critical difference to people's health, we want it to be part of clinical practice."
