It looks like from the paper's methods [0] that they did "sip and spit," maybe similar to wine tasting? Plus rinsing with normal water in between tastes. I thin

It looks like from the paper's methods [0] that they did "sip and spit," maybe similar to wine tasting? Plus rinsing with normal water in between tastes. I think that, plus D2O not being particularly poisonous in low quantities, would make that a safe enough experiment. In the same paragraph they say "All research procedures were ethically approved by the Committee for the Use of Human Subjects in Research" at their university, so they probably had to provide a lot of evidence that it was safe beforehand. [0] https://www.nature.com/articles/s42003-021-01964-y#Sec10 https://www.nature.com/articles/s42003-021-01964-y#Sec10