Yin fen is a strong, addictive substance made from diluted vampire venom.
Yin fen gives temporary relief for individuals in pain, giving the impression and false but seemingly satisfactory and pleasurable feeling of healing and euphoria, as well as a quick burst of strength.
A side effect of the drug is uncontrollable shaking of the hands, especially once the initial effects begin to wear off. Withdrawal from the drug causes weakness, pain, and anxiety in the addict. However, it is possible to wean off the drug by going cold turkey—abruptly getting off the drug and suffering through the pain—to allow the user to cleanse his/her system of the drug if done soon enough.
Yin fen is still a weaker and less powerful compared to pure vampire venom from a vampire's bite, though its effects last longer.
When Isabelle Lightwood was injured, Aldertree refused to let her go on a mission to the Adamant Citadel unless she had recovered. Aldertree tricked Isabelle into taking the drug, telling her that it would help heal the venom-infected wound at a much faster rate. The desperate Isabelle agreed, and Aldertree applied a small dose of it on her back; she quickly felt the effects and, as Aldertree gave her a whole case of it, quickly became addicted to it.
Aldertree continued to give Isabelle containers of yin fen and hung its availability over her head, knowing that she was becoming more and more addicted and dependent on it. At the Adamant Citadel, the presence of the drug in Isabelle's system made her fail the purity trial—the heavenly energy in the water detected the "demonic impurities" in her blood and would have destroyed her had Clary not interfered. Sister Magdalena informed her that the yin fen did not, and will not, heal her.