Drug-resistant TB occurs when TB bacteria become resistant to at least one of the main anti-TB drugs. The most concerning forms are Multidrug-Resistant TB (MDR-TB), which is resistant to both isoniazid and rifampicin, the two most potent TB drugs, and Extensively Drug-Resistant TB (XDR-TB), which is resistant to isoniazid, rifampicin, any fluoroquinolone, and at least one of the second-line injectable drugs.
