Reactivation of latent tuberculosis (LTB) has been described with the use of anti-TNFs. Combined treatment of isoniazid (INH) and disease modifying antirheumatic drugs (DMARDs) can potentially increase the risk of hepatotoxicity. The goal of this study was to investigate the risk of hepatotoxicity in rheumatic patients taking INH while on DMARDs and/or biologics. We reviewed the Institut de Rhumatologie de Montréal database (Rhumadata®) for rheumatic patients with positive tuberculin skin test or quantiFERON who took INH between August 2001 and April 2011. Liver function tests (LFTs) were collected at baseline and during therapy, and LFTs up to 9 months prior to INH initiation were used as controls. Of 922 patients screened for LTB, 87 patients tested positive. During INH treatment, 75.9 % were taking DMARDs, 82.8 % were taking biologics. A total of 375 LFTs performed while on INH were compared to 211 available tests collected prior to INH therapy. Twenty-four percent of the patients had abnormal LFTs during INH compared to 12.1 % prior to INH (p = 0.0481). Most of these abnormalities were mild or transient, but 8 % (seven patients) had significant abnormalities leading to INH discontinuation. Among these patients, mean (min, max) was 241 (52, 617) for AST and 262 (92, 669) for ALT. Although the use of INH therapy in combination with DMARDs and/or biologics was generally well tolerated, the rate of LFT abnormalities was higher when patients were exposed to INH, and significant abnormalities were more frequent than reported in the INH literature. It is prudent to closely follow the LFTs of these patients.