Dr Hilary warned Good Morning Britain viewers that mouthwash is not a miracle Covid-19 cure following a study which suggests it could reduce the risk of transmission.
Penn State University's research found products like Listerine can inactivate coronaviruses, although humans were not tested in the study.
Good Morning Britain host Susanna Reid asked Dr Hilary Jones if mouthwash could be effective in the fight against coronavirus.
She asked: "What is the evidence that mouthwash could work in some way?"
But Dr Hilary warned viewers that relying on mouthwash to beat Covid-19 is "dangerous".
He said: "Mouthwash might help as a virucidal, a bactericidal, to some extent.
"But whether it could actually penetrate deep into the lungs and nose where Covid is lurking, I think is a bit far-fetched, and I think we have to be cautious with this.
"I think it might help with sales of mouthwash right now, but I don't think we should rely on it."
While the study may prove useful in the future, it is too small to rely on now, Dr Hilary said.
He explained: "Well it's a small study, it hasn't been peer-reviewed, it's very preliminary.
"There are many companies jumping on a bandwagon saying mouthwash is a way to beat Covid, I think that's very dangerous."
