When the most commonly used drugs are effective against the disease, the patient is said to have drug susceptible TB, or standard TB. Drug-resistant TB (DRTB), on the other hand, is a strain of TB that presents resistance to one or more anti-tuberculosis drug. A more serious form of this is known as multi-drugresistant TB (MDR-TB), when patients are resistant to the two most powerful first-line antibiotics, isoniazid and rifampicin. Extensively-drug-resistant TB (XDR-TB) is a form of TB where the strains are, in addition, resistant to certain second-line drugs.