MDR (multi drug resistant) TB is the name given to TB when the bacteria that are causing it are resistant to at least isoniazid and rifampicin, two of the most effective anti TB drugs.

XDR (extensively drug resistant) TB is defined as strains resistant to at least rifampicin and isoniazid in addition to being resistant to one of the fluoroquinolones, as well as resistant to at least one of the second line injectable TB drugs amikacin, kanamycin or capreomycin.

MDR TB and XDR TB do not respond to the standard six months of treatment with "first line" anti TB drugs, and treatment for them can take two years or more and requires treatment with other drugs that are less potent, more toxic and much more expensive. Worldwide only a few thousand patients with MDR TB and XDR TB are treated each year.In areas of minimal or no multi drug resistant TB, TB cure rates of up to 95 per cent can be achieved. Cure rates for multi drug resistant TB are lower, typically ranging from around 50% to 70%.