Nytoxinol is a drug that causes tissue transplant rejection.
In 2372, Denara Pel, temporarily existing as a hologram, injected her real body with nytoxinol, causing her brain to reject the Klingon neural tissue that had been grafted to her cerebral cortex. Pel did this because she did not wish to return to her diseased body, even if it meant she would only live a few more days before her synaptic pattern degraded within the holobuffer. (VOY: "Lifesigns")