Radioactive isotope
From The Stargate Omnipedia
Isotope developed by the Tok'ra, capable of being injected into the body. It essentially "cloaks" the bio signs of an individual, hiding the recipient from Goa'uld sensors for up to eight hours.
KEY EPISODES
Fallen - Daniel Jackson and Jonas Quinn are injected with the Tok'ra radioactive isotope, allowing them to move freely aboard Anubis's mothership for eight hours, undetected.
Homecoming - The Tok'ra isotope wears of, making Jonas and Daniel visible to the internal sensors on Anubis's mothership.
Evolution, Part 2 - SG-1 and Jacob/Selmak infiltrate Anubis's base with radioactive isotopes to keep them undercover while they infiltrate the facility on Tartarus.