What is multidrug-resistant TB (MDR-TB)?

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Mycobacterium tuberculosis strains that are resistant to one or more anti-TB drugs have emerged.

Multidrug-resistant TB (MDR TB) occurs when bacteria become resistant to at least the two first-line drugs, Isoniazid and Rifampin. When people fail to complete treatment regimens or receive incorrect treatment, they may remain infectious.

Bacteria in their lungs may develop resistance to certain anti-TB drugs, which then can no longer kill the bacteria. People they infect will then acquire the same drug-resistant strain.

When drug treatment stops, the resistant bacteria will begin to multiply and cause active disease again, for which there are limited treatment options.

The end result is drug-resistant TB, a form of TB that doesn't respond to treatment.
Drug-resistant Mycobacterium tuberculosis strains may emerge due to:

  • Inconsistent or partial treatment
  • When patients do not take all their medicines regularly for the required period because they start to feel better
  • When health workers prescribe the wrong drugs or the wrong combination of drugs
  • When the drug supply is unreliable.

Drug resistance is more common in people who:

  • Have spent time with someone with drug-resistant TB disease
  • Do not take their prescribed medicine regularly
  • Do not take all their medicine
  • Develop TB disease again, after having taken TB medicine previously
  • Come from areas where drug-resistant TB is common (South East Asia, Latin America, Haiti and the Philippines)

People with MDR-TB disease must be treated with special drugs. These are not as effective as the usual anti-TB drugs and often cause severe side effects.

People with MDR-TB disease must consult a TB specialist to observe their treatment to check its effectiveness. 

MDR-TB is at least 100 times more expensive to cure than non-resistant TB. At best, only half those infected with new strains can be cured.

People who have spent time with someone with MDR-TB disease can become infected with MDR-TB bacteria. If they have a positive skin test reaction, preventive therapy is important for those at high risk of developing MDR-TB disease, such as children and HIV-infected people.

In 2013, the WHO reported that an estimated 480 000 peopled developed MDR TB. This occurred in 3.5% of patients who had never had TB before and in 20.5% of patients who had been previously treated.

In the same year, South Africa reported approximately 26 000 cases of laboratory-confirmed MDR TB; of these, 1.8% were new cases and 6.7% were patients who had been treated before.

The worst scenario is that TB will become untreatable due to the emergence of MDR-TB strains, with additional resistance to other drugs available for the treatment of TB. 

Read more: 

Risk factors for developing tuberculosis (TB) 

Treating tuberculosis 

What causes tuberculosis?

Reviewed by Dr AW Dreyer, Pathologist and Clinical Microbiologist, Centre for Tuberculosis, National Institute for Communicable Diseases February 2015.

Previously reviewed by Joanna Evans, PhD, Molecular Mycobacteriology Research Unit, Division of Medical Microbiology Institute of Infectious Diseases and Molecular Medicine, University of Cape Town, February 2011.

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