The invention relates to a stabilized premix feed or feed-like formulationthat has an extended shelf life due to a decrease of the degradates of theactive ingredient by controlling the amount of an already existing stabilizerin the formulation. The feed premix is used in the treatment or prophylaxis ofparasites in mammals, in particular swine and horses. The invention furtherrelates to a method to extend the shelf life of a stable premix feed or feed-like formulation for the treatment or prophylaxis of parasite infestation inswine and horses comprising controlling the amount of an already existingantioxidant or stabilizer in the formulation to decrease or to prevent theformation of acid/base catalyzed decomposition of the active ingredient.
