The 46-year-old man, from the town of Llançà in Girona, is a trained vet but was not currently practicing, according to Europa Press.
The seven-year-old boy was taken to Vall d'Hebron hospital in Barcelona when he suffered a reaction after ingesting the animal medicine.
He has been taken into care while an investigation is opened into his father.
The president of the Veterinary College of Girona, Bernat Serdà, said that the medicine used, Levamisol, had been discontinued in the year 2000 because of its adverse side effects.
Levamisol had been used in treating pigs and sheep with worms and other parasites. It is so far unclear why the father gave the medicine to his son.
The investigation continues.