With the increased number of military veterans returning from conflicts in Afghanistan and Iraq diagnosed with posttraumatic stress disorder (PTSD), there is a high vulnerability of these individuals to develop a substance use disorder (SUD). While there have been a host of studies focused largely on dopaminergic mechanisms of drug reward, they have not led to the development of adequate treatments for either preventing people diagnosed with PTSD from developing SUDs or for treating comorbid PTSD/SUDs. N-acetylcysteine (NAC) is an amino acid that may help to treat comorbid PTSD/SUD by increasing extracellular glutamate in the nucleus accumbens, a key brain structure in the craving circuitry. Restoring basal glutamate levels by pretreating with NAC may blunt the release of presynaptic glutamate. The aim of this studyis to establish a proof-of-concept trial protocol for measuring drug craving and relapse in Veterans suffering from comorbid PTSD/SUDs and to determine the efficacy of NAC in preventing relapse and reducing drug craving and PTSD symptoms among Veterans with comorbid PTSD/SUDs.