The Reol use a natural defense mechanism, a chemical produced in their bodies which resembles cortical acetylcholine, a neurotransmitter that helps regulate and process human memory. The chemical is spread through close proximity and dissipates fairly quickly, but it affects the temporal lobe associated with face recognition, allowing the mind to fill in details and to perceive the alien appearance of the Reol as quite human and familiar. Kaiael, a Reol encountered by SG-1 on P7S-441, used this defense mechanism to appear to SG-1 as Lieutenant Tyler, a fictitious member of the SGC.
SG-1 was able to obtain a sample of the Reol chemical for study, and Ren'al of the Tok'ra managed to synthesize a modified version of the chemical that can be used to disguise one's identity during undercover missions. The chemical is carried in a ring with a sharp point, and when worn on the hand it is possible to prick an enemy to administer a small dose of the chemical. The effect is almost instantaneous, causing the mistaken recognition of an individual as someone already familiar. Daniel first used the experimental chemical as a means to gain access to the summit of System Lords in the Hassara System. By using the drug on Yu, Daniel was able to impersonate Yu's trusted lo'taur, Jarren. The chemical was used again by Mitchell in an attempt to infiltrate the Lucian Alliance. To Netan, Mitchell appeared as Kefflin, one of his most loyal and elusive lieutenants, and Mitchell deceived Tenat into believing he was Netan.
Cross Reference: Jarren, Kaiael, Kefflin, Ren'al, Reol, Lieutenant Tyler
Episode Reference: The Fifth Man, Summit, Last Stand, Company of Thieves