The Department of Medical Sciences will study whether Ivermectin, a drug used to kill parasites in cows and horses, will be able to enhance Favipiravia's efficiency to treat Covid-19.
Laboratory studies have shown promising results on enhancing Covid-19 treatment.
The study will be done under close cooperation with Siriraj Hospital and other hospitals which will test the combined drug set on patients infected with the Sars-CoV-2 virus.
Hospitals involved in the research said results are expected soon.
Somsak Akkasilp, chief of the Department of Medical Sciences, said Ivermectin is used as a key drug to fight against Covid-19 in many South American countries, because Favipiravia is hardly used in those places.
However, results on the drug's efficiency are mixed, so there is no official recommendation to use it as prime drug to cure the disease from many international health bodies.
In Thailand, the department has not prohibited Ivermectin in Covid-19 patients under its medical treatment guidelines to Covid-19 patients, but it made a "remark" that the drug could be used under a doctor's advice.
"We don't recommend it being used as a main drug for Covid-19 patients," Dr Somsak said.
"India has stopped using it after finding it could not stop the disease.
"But our study will prove whether it could enhance Favipiravia's efficiency against the disease.
"We expect to see results in the next four months."
The World Health Organization said it has not yet approved Ivermectin to be a medicine for treating Covid-19, saying current evidence on the use of Ivermectin is inconclusive.
It recommends the drug only be used within clinical trials.
Meanwhile, Surachok Tangwiwat, deputy secretary-general of Food and Drug Administration, said the drug has not yet been approved for Covid-19 disease treatment.
It is now at a clinical trial stage.
He said laboratory tests show that it could kill the virus when the drug is used at a level of high intensity.
But such high-level doses may be harmful to health, so it is not allowed to be used in Europe.
He warned the public should not buy it for dealing with Covid-19, as it may be dangerous for the health.
According to the Thai FDA, there are 200 registered brands for the use in animals and only two brands for human uses.