Drug-resistant TB occurs when bacteria have developed resistance to at least one of the most common antibiotics used to treat it. There are two types of DR-TB — multi-drug resistant tuberculosis (MDR-TB) and extensively drug-resistant tuberculosis (XDR-TB). MDR-TB is resistant to at least two of the most common anti-TB drugs; XDR-TB is resistant to four or more. If left untreated, both types of DR-TB can cause serious health complications and even death.