N-acetyl cysteine (NAC) is a supplement form of cysteine. NAC is useful for various lung conditions and can help break down mucous. NAC can also be useful for treating conditions that involve oxidative stress such as degenerative myelopathy and chronic kidney disease in dogs. For cats, NAC can be useful in treating conditions that involve respiratory inflammation and also for FIV. Use caution with pets with stomach ulcers or pregnant animals.