I would watch if you worm them , to much may purge them, i did once take a ferret to the vetgs as it was skinny after severl weeks here of none stop food & to be honest the vet was scratching his head as to why, so he wormed them with pannacur paste, its like a syringe & you get a measured dose .
Not sure if drontal is tablet form but surely all wormers go by the animals weight as to how much to admimnister