Hill's Prescription Diet h d Canine Heart Care My Animal Dispensary N Acetyl L Cysteine, or NAC, is the acetylated form of the amino acid L Cysteine. It possesses strong antioxidant properties due to the presence of a thiol group ( SH). This form of cysteine is Acetyl L Carnitine Rooted Owl L Taurine & L Carnitine Cardio Support for Dogs, 60 ct. at Tractor Supply Co N Acetyl L Cysteine, NAC Supplement, 60 capsules Life Extension