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Tuberculosis treatment and its different regimens have scientific backgrounds for their formulations. To understand this, we need to know about the mode of action of each anti-TB drug first.


Mode of Action of Anti-TB Drugs

Anti-TB drugs have the following three actions:

  1. Early bactericidal activity: Killing of actively growing bacilli (in the phase of rapid multiplication and uninhibited metabolic activity).
  2. Sterilizing activity of persisting bacilli, i.e., metabolically inhibited organisms in a quasi-dormant state.
  3. Ability to prevent the emergence of drug resistance.

The ranking of first-line drugs with respect to their type of activity is indicated in Table 1 below.

Table 1: Ranking of first-line anti-TB drugs used in the treatment of drug-sensitive TB, based on the mode of action and activity

First-line Drugs Early Bactericidal Sterilizing Prevention of emergence of drug resistance
Isoniazid (H) ++++ ++ ++++
Rifampicin (R) +++ ++++ +++
Pyrazinamide (Z) ++ +++ +
Ethambutol (E) + Nil ++

Thus, each drug has unique characteristics and drug combinations will make the regimen more effective.


Need for Long Duration of Treatment of TB

  • Anti-TB drugs mostly kill actively multiplying tubercle bacilli.
  • When bacilli have low metabolic activity, i.e., when bacterial growth has almost come to a standstill and the organisms are “dormant”, they are not killed by otherwise bactericidal drugs. Such organisms are referred to as persisters*.
  • Though they may survive in the presence of drugs, behaving as if they were drug-resistant, they are in fact susceptible to the drugs.
  • Thus, if for some reason these organisms regain their ability to multiply freely, they would be killed by the very drugs that had not harmed them before.
  • When dormant bacilli again become metabolically active and start multiplying during effective chemotherapy, they are soon killed.
  • Once chemotherapy has been completed, the revived bacilli may continue to multiply and thus cause relapse.
  • This explains why conventional chemotherapy needs to be of long duration.





Question​ Answer 1​ Answer 2​ Answer 3​ Answer 4​ Correct answer​ Correct explanation​ Page id​ Part of Pre-test​ Part of Post-test​
What is the role of the intensive phase of anti-TB treatment? To reduce adverse drug reactions in patients

To achieve rapid killing of actively multiplying bacillary population


To prevent the emergence of drug-resistance Options 2 and 3 4 The role of IP is to achieve rapid killing of actively multiplying bacillary population and eliminate naturally occurring drug-resistant mutants and prevent the further emergence of drug resistance.   Yes Yes
Which of the following drugs is bacteriostatic? Isoniazid Ethambutol Pyrazinamide Rifampicin 2 Ethambutol is an effective bacteriostatic drug, helpful in preventing the emergence of resistance to other companion drugs.   Yes Yes


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