The primary reason for the use of drug combination in the treatment of tuberculosis is
Ref-KDT 6/e p745 The bacteria that cause tuberculosis (TB) can develop resistance to the antimicrobial drugs used to cure the disease. Multidrug-resistant TB (MDR-TB) is TB that does not respond to at least isoniazid and rifampicin, the 2 most powerful anti-TB drugs. ... In some cases even more severe drug-resistant TB may develop.
Ensure patient compliance with the drug regimen
Enhance activity against metabolically inactive mycobacteria
Delay or prevent emergency of resistance
Provide prophylaxis against other bacterial infection
Other topics and Adverse effects