He is best known for his part in what historian of religion Ioan Culianu called "one of the most famous cases of demonic possession in the seventeenth century" where, in 1618, a young widow of Nancy, France, Elisabeth de Ranfaing, fell into the hands of the local doctor, Charles Poirot, who allegedly proceeded to violate her and give her medicine - intending to cause demonic possession.
Pithoys was called to perform an exorcism but refused and instead wrote his Descouverture des faux possedez (1621) where he attacked the doctor's evidence against Elisabeth and the doctor himself, claiming he had drugged her into convulsions and insanity - simulating demonic possession.
This was displeasing towards the local clergy and so a less skeptical doctor, Remy Pichard, was brought in to exorcise Ranfaing.
The doctor was subsequently burned at the stake in 1622 and Elisabeth was fully exorcised in 1625, founding a religious order in later life.
[3][4] Jean Lhermitte and Étienne Delcambre argued this tale must be apocryphal, as the effect of the doctor's drug would not be likely to persist for the seven years for which Elisabeth was possessed.