Engineered muscle n-acetylcysteine" refers to a product containing N-Acetyl-L-Cysteine (NAC), a supplement sold by the brand Engineered Muscle to support athletes and bodybuilders. NAC is a precursor to glutathione, an important antioxidant, and is believed to reduce oxidative damage, support muscle recovery, and protect the liver from stress caused by intense training and performance-enhancing drugs (PEDs).