The Upir Toxin, also described as Upir Neurotoxin, is a highly degenerative venom or enzyme found in Upirs, which is powerful enough to kill someone of their own kind. It is shown in the form of excess sinus in the maxilla of the mouth and created inside venom glands. It is only able to negatively affect other Upir, as not many humans bitten by an Upir in the show have shown any signs of venom. The enzyme can be used for medicinal purposes, breaking down the immune system and weakening an Upir's cellular defenses to allow for further treatments of they experience certain rare diseases or afflictions.
It was used in Olivia's recovery after she was killed by Roman at the end of season 1, as she could not recover on her own. It was said to have almost killed her in the process. It also caused her to become more human, giving her tissue dystrophy, chemically enhanced emotions and empathy, which was hard to overcome, accelerated aging and decay and even human like vulnerability to cancers, which was a possible side effect of the accelerated aging. Olivia had severe muscular dystrophy after being drained of blood by Roman and being given the Neurotoxin in her treatments, all of which caused her to become so weak that she needed a cane to walk which she got over in months following. She took medication such as methotrexate, cyclophosphamide, carboplatin, and etoposide. Interestingly, the chemical only did not remove her enhanced strength, biological immortality, or instinctual aggression, while it did stunt her healing and overall recovery abilities. This could all be immediately reversed if an afflicted Upir feeds off a Humanoid grown at the Godfrey Labs.
It is also the primary weakeness of the Jarmungandr, which are sulfur-based life forms, as it has an extremely high concentration of nitrogen oxide, with exposure causing a robust reaction capable of incinerating them.