TB can become resistant to the two most powerful medicines that we have to treat it - isoniazid and rifampicin. If a TB infection is no longer able to be treated with either of these drugs, it is classed as multi-drug resistant TB.     Multi-drug resistant TB is a big problem. Normally treatment for TB is 90% effective, but with drug resistance, this drops to as low as 50%.     Multi-drug resistant TB is more likely to develop when a person does not adhere to treatment. Where drug-resistant TB is common in a community, it's possible to be infected with a strain of TB that's already got some resistance before you've started treatment.