Persona Suppressors (ペルソナ制御剤) are a drug featured in the Persona series. Their common function is to keep unruly Personas from killing their hosts but ironically have lethal side effects.
Profile
Persona 3
Persona suppressors were used by Strega as well as by Shinjiro Aragaki. Stopping the use of the drug could result in a user's Persona killing them. The reason for this is that the members of Strega awakened their Personas through unnatural means. According to an answer to a fan question sent to the Persona Stalker Club, they can also cause the body to become unable to regulate body temperature, hence why Shinjiro is always wearing a coat and hat.
Persona -trinity soul-
Persona suppressors were used by Marebito as well as Ryo Kanzato. While the overall function of the suppressors were the same, Marebito required them because their Persona had consumed too many other Persona, causing them to go berserk. Additionally, Persona were explained to have an age limit, so older users lost control of their Persona and required suppressors if they wanted to continue to summon them.