A powerful medication given to protect a person against severe theta radiation exposure, which Janeway requested for her intended solo mission in The Omega Directive.
Arithrazine is a treatment for heavy exposure to theta radiation, and it is potent enough that regulations require a doctor to supervise anyone who takes it.
In the Voyager episode The Omega Directive, Janeway is under secret orders to destroy the Omega molecules the ship has detected. Her first plan is to go in a shuttle with Seven of Nine, and she asks the Doctor for a strong dose of arithrazine without telling him why. He points out the rule about supervision and is told that she cannot explain.
The odd request is one of the first signs to the senior officers that their captain is acting under a directive they have never heard of. Chakotay eventually persuades her to bring the crew into her confidence.