We experimented with the blood of sire werewolves. And we found it was possible to reverse the early stages of lycanthropy... in rodents.
For centuries, hunters believed that lycanthropy had no cure, until Sam read up on lore books from the British Men of Letters. The cure for lycanthropy, referred to as "plasma therapy", was first theorized by the BMOL in the 1930's. Their experiments used the blood of sire werewolves to successfully reverse the early stages of lycanthropy in one out of nine mice -- an 88% fatality rate. The therapy was attempted on a human once, causing the subject to die in agony. After the failure of the human trial, research into the cure was abandoned in favor of killing werewolves outright.
The ingredients to the cure are unknown outside of the blood of the sire werewolf. It appears live blood is required for the cure to be effective, unlike the cure for vampirism which required the blood regardless of whether the sire is still dead or alive. Mick Davies says the cure also depends on the infected refusing to feed, as feasting on a human heart will make the transformation permanent. Much like the cure for vampirism, the infected go through an agonizing ordeal as the cure reverses the transformation, ending when their claws and teeth retract and their eyes return to normal if they manage to survive the reversal process.
Some hunters, including John Winchester, theorized that "severing the bloodline" -- killing the werewolf that bit them -- would cause the person to revert back to a human, however this theory was disproven in 2.17 Heart.
While reading through Mick Davies' lore books, Sam discovers that the British Men of Letters were working on a treatment for werewolves known as "plasma therapy" in the '30s. Mick, however, tells Sam that the experiments were useless.
After Claire gets bitten by the werewolf Justin, Mick elaborates to Sam and Dean that they experimented with the blood of sire werewolves and discovered that it was possible to reverse the early stages of lycanthropy in rodents. However, only one out of nine mice survived the treatment and when it was tested on a human, the subject died an agonizing death, resulting in the project's abandonment. With no other options, Sam and Dean attempt to use the cure. Mick tells them they need live blood from the werewolf that bit Claire. As Sam and Dean track the man they believe to be the werewolf, a bartender named Conner, they find that he is human; meanwhile, Justin barges into her hotel room and takes Claire away.
After tracking Justin to his home, Mick is able to stick him with a blood collection tube and later shoots and kills Justin as he fights with Sam and Dean. Mick takes a syringe with the cure prepared in advance and mixes the sire werewolf blood into it and injects it into Claire's shoulder. After the cure is administered, Claire begins going through the agonizing process of becoming human again. The cure stops her heart and restarts it after her claws and fangs retract and her eyes turn back to normal, making her the first person successfully cured of lycanthropy.