What Does N-A-C Sustain Do? NAC Sustain is in a special Sustain bilayer Quick Release/Sustained Release format of N-Acetyl-L-Cysteine (NAC), a powerful antioxidant amino acid and a precursor to the critical antioxidant glutathione. Glutathione exerts a variety of protective effects, including detoxification and intracellular defense against oxidative stress. Substances known to reduce glutathione status are acetaminophen and alcohol.