Inappropriate or incorrect use of anti-TB drugs can cause drug resistance. This means that initially drug sensitive TB is treated with only one or two drugs or the treatment is too short or the treatment has been interrupted many times. Disease caused by resistant TB bacteria fails to respond to conventional, standard anti-TB drugs. Resistant tuberculosis may be transmitted in a similar way as drug sensitive TB.
Multidrug-resistant tuberculosis, MDR-TB, is a form of TB caused by bacteria that do not respond to isoniazid and rifampicin, the two most powerful, first-line anti-TB drugs. Extensively drug-resistant TB, XDR-TB, is a form of multi-drug resistant tuberculosis that responds to even fewer available medicines, including the most effective second-line anti-TB drugs.
Multidrug-resistant TB is found all over the world. More than half of these cases are in reported in India, China and the former Soviet Union. In Finland there are under ten new drug resistant TB cases annually.