acetylcysteine (NAC) is most commonly used for the treatment of acetaminophen overdose and acetaminophen-induced liver injury. In patients infected with , the causative agent of tuberculosis (TB), NAC is given to treat hepatotoxicity induced by TB drugs. We had previously shown that cysteine, a derivative of NAC, potentiated the activity of isoniazid, a first-line TB drug, by preventing the emergence of INH resistance and persistence in in vitro. Herein, we demonstrate that in vitro, NAC has the same boosting activity with various combinations of first- and second-line TB drugs against drug-susceptible and multidrug-resistant strains. Similar to cysteine, NAC increased respiration. However, in -infected mice, the addition of NAC did not augment the activity of first- or second-line TB drugs. A comparison of the activity of NAC combined with TB drugs in murine and human macrophage cell lines revealed that studies in mice might not be recapitulated during host infection in vivo.