I think that's more than likely here as "placebo" was literally this peppermint cordial [1].
They say it doesn't have peppermint oil in it... but it does list "flavorings" and I don't know how you make something taste like peppermint without using peppermint (or why you wouldn't).
The other possible explanation is 1 group is basically drinking an extra 200ml of water with a microscopic amount of peppermint oil, and the other group is drinking an extra 200ml of sugar water. The researchers may have literally just stumbled on the fact that more water and less soda has positive health impacts.
