Depression
Depression is a state of low mood and aversion to activity that can affect a person's thoughts, behavior, feelings and sense of well-being. Depressed people feel sad, anxious, empty, hopeless, worried, helpless, worthless, guilty, irritable, hurt, or restless. They may have problems concentrating, remembering details, or making decisions, and may contemplate, attempt, or commit suicide.
In the context of hallucinogen usage, this effect is triggered by certain substances as a temporary effect which can be considered as the polar opposite of euphoria.