Deuterium poisoning was an illness caused by exposure to refined deuterium. Its symptoms included headaches, blurred vision, nausea, and hallucinations; these effects could be exacerbated by lack of sleep. There were medications to alleviate the symptoms, while the condition itself was treated by simple rest.
In 2259, Doctor Joseph M'Benga diagnosed Ensign Nyota Uhura with mild deuterium poisoning after she reported seeing a gruesome apparition of a deceased Hemmer in the turbolift. Uhura had indeed been exposed to a small amount of refined deuterium while working inside a nacelle, but after consulting with Lieutenant James T. Kirk, who had suffered deuterium poisoning before and experienced nothing like her visions, it was eventually discovered that she was receiving communication from extradimensional lifeforms. (SNW: "Lost in Translation")