What Causes Diarrhea After Having A Meal In A Toddler?

Hi Doctor, My son 2.6 years old, and he had a problem of loose motions , after every meal and especially wenever he drinks milk from last 2months .But the problem of loose motions is since birth. Aafter few test doctor said he is allergic to milk, so we have to stop his milK and milk products. And he prescribed Neogadine mikros , aristozyme liquid and zincovit syrup ( 5ml BD) for 3months. His loose motions are controlled bUT MY CONCERN, is it right to give these medicines soo long , their must be some kind side effects and might be his stomach addicted to these medicines. pls answer abt dis