TORONTO -- A low-cost, widely available drug called dexamethasone is being hailed as a promising new treatment for COVID-19 after preliminary clinical trial results out of the U.K. showed it could reduce deaths by up to one third in critically ill patients.

On Tuesday, researchers from the University of Oxford announced their initial findings from a large randomized clinical trial they conducted to test a range of potential treatments for COVID-19, including low-dose dexamethasone.

The drug was given orally and through an IV to 2,104 randomly assigned COVID-19 patients. They were compared to 4,321 other patients receiving regular treatment for the disease.

After 28 days, there was a reduction in deaths by one third in ventilated patients and by one fifth in patients who only needed supplemental oxygen. There appeared to be no benefit for patients with milder disease.

Without an available vaccine for COVID-19, dexamethasone is being embraced by government and health officials around the world as a hopeful alternative to save lives thanks to these initial findings.

On Wednesday, the director-general of the World Health Organization (WHO), Dr. Tedros Adhanom Ghebreyesus, applauded the U.K. researchers’ work during a virtual briefing from Geneva.

“This is very welcome news for those patients with severe illness,” he said. “This drug should only be used under close clinical supervision. We need more therapeutics that can be used to tackle the virus, including those with milder symptoms.”

In an earlier statement, Ghebreyesus congratulated the U.K. government, the University of Oxford, and the participating hospitals and patients who contributed to “this life-saving scientific breakthrough.”

British Prime Minister Boris Johnson echoed that sentiment when he called it the “biggest breakthrough yet” in treating coronavirus. The U.K. government approved dexamethasone for the immediate treatment of all hospitalized COVID-19 patients requiring oxygen.

In the U.S., top infectious disease expert Dr. Anthony Fauci described it as “a significant improvement in the available therapeutic options that we have.”

Here’s what we know about dexamethasone and how it could be used to treat COVID-19.

WHAT IS DEXAMETHASONE AND WHAT IS IT USED FOR?

Dexamethasone is a commonly used corticosteroid used to reduce inflammation in a range of conditions, including inflammatory disorders, immune system problems, and certain types of cancers.

It’s also been used to treat severe allergies, adrenal (small glands located on top of the kidneys) problems, arthritis, asthma, blood or bone marrow problems, kidney problems, skin conditions, and flare-ups of multiple sclerosis, according to the U.S. Mayo Clinic.

Dexamethasone is similar to cortisol, a naturally occurring anti-inflammatory hormone produced by the adrenal glands. When the body is not making enough cortisol, patients are given dexamethasone as a substitute, according to the U.S. National Library of Medicine.

It can be prescribed in tablet form or as an oral solution.

The WHO says the drug has been used as a treatment for various conditions since the 1960s and it has been included on the health body’s Model List of Essential Medicines in multiple formulations since 1977.

HOW COULD IT TREAT COVID-19 PATIENTS?

While the majority of COVID-19 patients appear to be asymptomatic or only experience mild symptoms such as cough and fever, there are some who become seriously ill as a result of the infection.

In these cases, scientists say the patient’s immune system goes into overdrive and ends up attacking their own organs and cells in an attempt to fight off the virus. This overzealous immune system response is often referred to as a “cytokine storm.”

That’s where dexamethasone comes in. The steroid is used to reduce inflammation and suppress the immune system in cases where it’s not functioning properly.

According to the U.K. researchers involved in the clinical trial, dexamethasone was the most effective for critically ill COVID-19 patients who required a ventilator to breathe.

Dr. Michael Ryan, the executive director of the WHO Health Emergencies Programme, stressed that dexamethasone should only be used in COVID-19 patients who are in this condition.

“Steroids, particularly powerful steroids, can be associated with viral replication. In other words they can actually facilitate the division and replication of viruses in human bodies,” he said during the virtual briefing on Wednesday

“It's exceptionally important in this case, that this drug is reserved for use in severely ill and critical patients who can benefit from this drug clearly.”

ARE THERE SIDE EFFECTS?

Common side effects of dexamethasone include anxiety, insomnia, headache, upset stomach, vomiting, dizziness, depression, and increased hair growth.

Rarer side effects for the drug include vision problems, skin rash, muscle weakness, and swollen face, lower legs, or ankles, according to the U.S. National Library of Medicine.