For the prevention of parasite infestation of animals an isoxazoline can be administered by drinking water route. However the inventors have found that when the drinking water is sanitized for instance by using hypochlorite the isoxazoline becomes degraded. Surprisingly the isoxazoline can be protected from degradation by the use of a vitamin E. A pharmaceutical composition can now be prepared containing a concentrated solution of the isoxazoline in a solvent and co solvent with vitamin E. The composition can be diluted in drinking water even when sanitized to prepare medicated drinking water for animals. This way an anti parasitic treatment can be mass administered leading to a highly effective reduction of the parasite infestation of an animal and its surroundings.
