Multidrug-resistant TB (MDR-TB) is defined by resistance to the two most commonly used drugs in the currentfour-drug (or first-line) regimen, isoniazid and rifampin.MDR-TB has emerged and spread due to the inadequacy oftreatment. Today, treatment for drug-resistant TB can takeup to two years, and is so complex and toxic that a thirdof all MDR-TB patients die.