If TB-infected people do not take their medicine as their doctor or nurse directs, the TB bacteria may become resistant to certain medicine. This means that the medicine can no longer kill the bacteria.
Drug resistance is more common in people who:
Have spent time with someone with drug-resistant active TB disease
Do not take their medicine regularly
Do not take all of their medicine as told by their doctor or nurse
Develop active TB disease again, after having taken TB medicine in the past
Come from areas where drug-resistant TB is common
Sometimes the bacteria become resistant to two or more of the most important TB medicines. This is called multidrug-resistant TB, or MDR TB. This is a very serious problem. People with MDR TB disease must be treated with special medicines. These medicines are not as good as the usual medicines for TB and they may cause more side effects. Also, most people with MDR TB disease must see a TB expert who can closely observe their treatment to make sure it is working.