Hello Doctor, My 2 Year Old Has Been Diagnosed With

Hello Doctor,
My 2 year old has been diagnosed with enlarged adenoids. Pediatrician has prescribed pentids 200 course, 3 times a day for 3 months. Is it safe to give the antibiotic for that long? Also what could be the possible side effects as the medication has started for a week now and I have observed mild diarrhea and excess greenish discharge from nose. Please guide. Thank you