My oldest son got thrush two summers ago when he was 6. My one year old had thrush at the time and he was in the habit of picking up toothbrushes that were laying around and putting them in his mouth to chew on. My 6 year old was always leaving his toothbrush laying around. So, before we knew it, he had thrush too. The nystatin that the doctor prescribed DID work on him, though it did not work on the baby. The baby and I kept passing it back and forth and it did not go away on him until I used gentian violet. But, once my oldest started the nystatin, his thrush cleared up pretty quickly and we learned to put his toothbrush up where baby brother couldn't get it - lol.
My DS is in the last week of his prescription of nystatin. We have been really careful about sterilizing his bottles and nipples. The nystatin seems to be working except I am horrible at giving medication. But when my DS first got thrush it was extremely painful for him. After about two days of the meds, his pain level decreased tremendously.