PerfectValbazen cattle and sheep wormer is a very safe wormer for chicken. Dosage is 1/2 ml for an average 5 pound hen, given orally, and repeated in 10 days. It is available in aome feed stores, and online.
SafeGuard Liquid Goat Wormer is usually found in most feed stores. Dosage is 1/4 ml per pound (1.25 ml for 5 pound hen) given orally for 5 straight days. Have you had your cet do a fecal float? Or have you seen any worms in the droppings?