Dermal osmotic sealant was a medication used to protect humanoids from high levels of trigemic vapors.
In 2372, The Doctor whipped up a batch in anticipation of an USS Voyager mission to a planet with such vapors, nicknamed planet Hell. (VOY: "Parturition")
Later in the episode, Neelix and Tom Paris are seen using sprays against the vapors which according to Paris, The Doctor told would help. This was presumably intended to be the sealant.