Effective
My cat, Titi, developed severe allergies from when she was 5 months old (diarrhea, weight loss, lack of appetite, apathy, etc.) We found out that this was life-threatening, so we started feeding Z/d.
Ever since, Titi only feeds on it. He now is 2 years old and absolutely thriving and has finally reached a normal bodyweight.
Yes, it is expensive, but health is a priority!