The blue listerine that the Thai prostitutes use is to reduce STIs
They don’t use it to make their breath smell good.
One such study, published in the journal Sexually Transmitted Infectionsin 2017, looked at whether gargling with Listerine could be an effective way to reduce the risk of oropharyngeal gonorrhea (gonorrhea of the mouth or throat) among males who had sexual contact with other males within the previous 12 months.1
What the researchers found was that the male participants—all of whom had N. gonorrhoea in their mouths, as evidenced by oral swabs—were 38% less likely to have any trace of the bacteria after gargling with Listerine compared to those who gargled with plain salt water.1