The most common medicine is Corid or amprollium, and it is very safe. It won’t hurt them if they don’t have coccidiosis. Most feed stores sell it in the cattle medicines. Dosage is 2 tsp of the liquid Corid or 1.5 tsp of the powder per gallon of water for 5-7 days. Change it daily. Give it to all of them. How old are the chickens and what are the symptoms in the one chicken?
Can I find it on Amazon? She is a silky named sandy, today I've noticed a few of her poops had red, like blood in it. But she seems to be eating and drinking ok so far. She is about 4 months old.