Lucovextrin was a substance that wasn't normally picked up by the sensors of a Galaxy-class starship. This substance was used as a dietary medication in the Central Plains Area of the planet Qo'noS by Klingon women, until the KDA determined it to be too toxic and banned its use.
Furthermore, along with invidium, saltzgadum, selgninaem, and jakmanite, it was one of only five undetectable substances that can exist in a nitrogen-oxygen atmosphere and can alter the molecular structure of glass.
Lucovexitrin was highly toxic and was thereby ruled out as having caused the malfunctions aboard the USS Enterprise-D at the end of 2366, as the whole crew would have died of poisoning. (TNG: "Hollow Pursuits")