@ERSpublicationsThe classification of the anti-TB drugs should probably be modified to optimise the use old and new compounds http://ow.ly/QbDSP Multidrug-resistant (MDR) tuberculosis (TB) (resistance to at least isoniazid and rifampicin), with >480 000 cases in 2013, 10% of them being affected by extensively drug-resistant (XDR)-TB (MDR-TB with additional resistance to any fluoroquinolone, and to injectable second-line drugs (SLDs) (capreomycin, kanamycin or amikacin)), continues to represent a real threat to TB control [1][2][3][4]. In some high MDR-TB burden countries, the prevalence of MDR-TB among new cases exceeds 20% and among retreatment cases, reaches almost 50% [1,5].Prevention and quality diagnosis and treatment of MDR-and XDR-TB are part of the crucial interventions included in Pillar 1 of the new World Health Organization (WHO) End TB Strategy, which is focussed on the goal of TB elimination [6][7][8]. Unfortunately, the outcomes of these cases are suboptimal, with 60% success among MDR-TB cases [2], 40% among XDR-TB cases [3] and <20% among the cases with resistance patterns beyond XDR-TB [4]. This means not only high mortality but also significant human suffering and transmission of the Mycobacterium tuberculosis strains within the community [9]. Unfortunately, at present, the treatment of MDR/XDR-TB is still very long, toxic and expensive [10,11].The original WHO guidance on the management of MDR-TB was issued in 1996 [12] and since then, several updated guidelines have been published. The fundamentals on MDR/XDR-TB treatment were published in the 2006 and 2008 WHO guidelines [13,14], where the basic rationale to design therapeutic regimens for these cases was described. According to these recommendations, which are currently followed worldwide, the anti-TB drugs are classified into five main groups indirectly considering the evidence available, the safety and/or the effectiveness of the different drugs. These groups start from the most effective drugs (group 1), grouping the other drugs progressively according to decreasing order of efficacy [15].In the 2011 revision of the WHO MDR/XDR-TB guidelines [16], the composition of different groups of anti-TB drugs was not modified. Therefore, since 2006, the rationale of the classification of the anti-TB drugs has not been changed but several studies have examined the effectiveness, efficacy and safety of the group 5 drugs, and some of these drugs appear to be more effective and well tolerated than others that currently have "higher" ranking in the classification. Furthermore, promising new drugs active against M. tuberculosis have been discovered in the last couple of years [17][18][19][20] and have become progressively available in many parts of the world to treat MDR/XDR-TB.