In recent years, new strains of tuberculosis (TB) bacteria have developed antibiotic resistance to one or more of the primary medications typically used to treat the disease. The disease caused by these antibiotic-resistant bacteria is called multidrug-resistant tuberculosis (MDR TB).

The main cause of MDR TB is partial or inconsistent antibiotic treatment. Because treatment of TB may involve taking medications for several months to a year after symptoms have disappeared, it is easy for people to forget or decide to no longer take their medications. Also, people who do not have the money for or access to adequate health care may only receive short-term or sporadic treatment. This allows the TB bacteria to become resistant to the medication used to treat it.

A patient with MDR TB can transmit the disease in its antibiotic-resistant form, potentially creating new cases of MDR TB that will also be difficult to treat.

Hospitalized patients have an increased risk of contracting MDR TB secondary to the condition for which they were hospitalized (a nosocomial infection). This is because many patients who are hospitalized have weakened immune systems, making them more susceptible to infection. In addition, many of the strains of TB bacteria common in hospitals (and to which patients may be exposed) are already antibiotic-resistant.

MDR TB is difficult to successfully treat. It may not be recognized until months after a patient has started taking the initially prescribed antibiotics, wasting valuable treatment time. People with MDR TB are usually prescribed a longer course of treatment – often up to two years – and treatment may include second-line drugs. These second-line drugs are not suitable for everyone (e.g. children, pregnant women) because of their serious side effects. MDR TB is especially dangerous for patients with HIV or AIDS, for whom TB is often fatal within months of diagnosis.

Recently, a more deadly strain of TB, called extreme drug-resistant TB (XDR TB), has emerged. XDR TB is resistant to at least three, and sometimes up to six, of the antibiotics commonly used to treat TB, making it virtually impossible to treat. According to the World Health Organization (WHO), 4 percent of MDR TB cases in the United States are actually misdiagnosed cases of XDR TB. In some countries, especially those in the former Soviet Union and Africa, almost 20 percent of MDR TB cases are really XDR TB. 

Despite the fact that the overall rate of TB in the United States has steadily declined over the last 10 years, cases of MDR TB are increasing. According to the U.S. Centers for Disease Control and Prevention (CDC), the number of MDR TB cases increased by more than 13 percent in 2004. It is thought that many cases of MDR TB may turn out to be XDR TB.