From the Manufacturer:
N-acetyl cysteine is the acetylated form of L-cysteine, which is more efficiently absorbed and used. It is also an antioxidant that is helpful against viruses. N-acetyl cysteine has been used as a liver protectant and to break up pulmonary and bronchial mucus. N-acetyl cysteine also boosts the levels of glutathione, a powerful antioxidant.