powder can be stored in a closed container at room temperature (away from high heat or humidity) for at least two months, and possibly longer, without spoiling or degrading. Small amounts of NAC may degrade, over longer periods of time, into a by-product called di-NAC — however, this substance is not known to be harmful. (The sulfur in NAC may cause a "rotten egg" odor, but this does not indicate a problem.) If you are concerned about degradation, consider capsules or tablets, as they may be more stable than powder.
See ConsumerLab's Review of NAC (N-Acetyl-Cysteine) Supplements
for our tests of popular NAC supplements and our Top Picks
among products, including cost comparisons and information about where to buy NAC.