Two over-the-counter drugs have been found to inhibit the replication of SARS-CoV-2, the virus that causes COVID-19, in preliminary tests, according to a study.

Researchers from the University of Florida in the US noted that the combination includes diphenhydramine, an antihistamine used for allergy symptoms.

When paired with lactoferrin, a protein found in cow and human milk, the compounds were found to hinder SARS-CoV-2 during tests in monkey cells and human lung cells, they said.

Lactoferrin is commonly used as a supplement to treat stomach and intestinal ulcers, among other uses, according to the researchers.

"We found out why certain drugs are active against the virus that causes COVID-19. Then, we found an antiviral combination that can be effective, economical and has a long history of safety," said David A Ostrov, an associate professor at the University of Florida.

In lab tests on human and monkey cells, the combination of drugs was particularly potent, the researchers said.

Individually, the two compounds each inhibited SARS-CoV-2 virus replication by about 30 per cent, they said, adding that together, they reduced virus replication by 99 per cent.

The findings, published in the journal Pathogens, are a first step in developing a formulation that could be used to accelerate COVID-19 recovery.

Additional research into the compounds' effectiveness for COVID-19 prevention is already underway in mouse models, the researchers said.

To establish their findings, they focused on proteins expressed in human cells known as sigma receptors.

In COVID-19 cases, the virus "hijacks" stress-response machinery, including sigma receptors, in order to replicate in the body, according to the researchers.

Interfering with that signalling appears to be the key to inhibiting the virus's potency, they said.

"We now know the detailed mechanism of how certain drugs inhibit SARS-CoV-2 infection," Ostrov said.

Data from the experiments show that a highly specific sigma receptor binding drug candidate and formulated combinations of over-the-counter products have the potential to inhibit virus infection and decrease recovery time from COVID-19, the researchers said.

While the findings are encouraging, Ostrov cautions against self-medicating with either diphenhydramine or lactoferrin as a COVID-19 prevention or treatment.

The type of lactoferrin used in the research differs slightly from the type that is commonly available to consumers, he added.