The Reol chemical is a substance that affects the mind of people.
The chemical is a substance specifically designed to infect the mind of an intended person and trick them into seeing whomever or whatever the user says. It is produced by the Tok'ra and derived from the natural secretions of the Reol species, the chemical being stored in a ring-like device that the user can wear discreetly. It is also strongly advised that the wearer does not accidentally prick themselves, as the effects of doing so are unknown. The chemical must be used carefully, as the ring typically only holds enough for a single dose, requiring potential spies to choose the person that they will be impersonating very carefully.
Daniel Jackson used the chemical in March 2002 to infiltrate a summit of the Goa'uld System Lords by pretending to be Yu's Lo'taur, as none of the other System Lords would have met Daniel or the Lo'taur before, and the mission required someone who spoke fluent Goa'uld without possessing a symbiote. (SG1: "Summit")
Six years later, in September 2006, Cameron Mitchell used this chemical to infiltrate the Lucian Alliance and pretend he was Kefflin, a high-ranking but reclusive member of the Alliance who was only known by sight to Netan (although the ring retained enough of a dose for Mitchell to later convince a bounty hunter that he was Netan, and trigger an inter-ship conflict so that SG-1 could escape). (SG1: "Company of Thieves")