The shares were down 4 percent on Wednesday, but more than 1.2 million tweets had employed the hashtag #Isojin by afternoon.
As governments worldwide have struggled to rein in the virus, which has killed nearly 700,000 people and infected more than 18 million, some in authority have seized on any treatment seeming to offer a glimmer of hope.
For example, US President Donald Trump has persistently promoted the unproven drug hydroxychloroquine as successful in treating the virus, and in April, he asked publicly if injecting disinfectant might help.
Read also: Two Indonesian Men Summoned by Police for Touting Covid-19 Cure
Japan's top government spokesman, Yoshihide Suga, said he was aware of the Osaka governor's comments, including the fact that researchers would try to determine if the ingredient could keep disease symptoms from worsening.
"The government will closely watch developments of the research," Suga told a news conference.
Some experts were skeptical, however.
"I think these kinds of claims might even lead to a high number of false negatives for PCR tests," pharmacist and medical writer Shuichi Aoshima wrote on Twitter, adding that tests after use of the germicide would register lower levels of the virus.
"It's the same as dripping povidone-iodine onto a virus sample."
(Writer: Sakura Murakami, Chang-Ran Kim | Editor: Clarence Fernandez)
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