Arithrazine was a medication used for the most extreme cases of theta radiation poisoning. According to Starfleet Medical protocols, a doctor had to monitor the patient who had received arithrazine during treatment.
When the USS Voyager encountered the Omega molecule in the Delta Quadrant in 2374, Captain Janeway intended to go out and destroy the molecule after receiving arithrazine by The Doctor. As he could not monitor her, he initially refused to give the medication, but she overruled him with the Omega Directive. The arithrazine was later used to inoculate rescue team members on a mission to destroy the Omega molecules. (VOY: "The Omega Directive")