BACKGROUND Treatment of TB is often extended beyond the recommended duration. The aim of this study was to assess prevalence of extended treatment and to identify associated risk factors. We also aimed to determine the frequency and type of adverse drug reactions (ADR) experienced by this study population.METHODS We performed a retrospective cohort study of all patients treated for active TB at Christchurch Hospital, Christchurch, New Zealand, between 1 March 2012 and 31 December 2018. Data for 192 patients were collected on patient demographics, disease characteristics and treatment characteristics, including planned and actual duration of treatment and ADRs.RESULTS Of 192 patients, 35 (18.2%) had treatment extended, and 85 (46.5%) of 183 with fully drug-susceptible TB received ≥9 months treatment. The most common reasons for extension were persistent or extensive disease and ADR. Extended treatment duration was not associated with any patient or disease characteristics. We found 35 (18.2%) patients experienced at least one ADR. The most common ADRs were hepatitis, rash and peripheral neuropathy.CONCLUSION TB treatment extension beyond WHO guidelines is common. Further research is needed to guide management of those with slow response to treatment. Methods for early detection of ADR, systems to improve adherence and therapeutic drug monitoring are potentially useful strategies.