A 30-year-old mother of 2 children weighing 60 kg was taking combined oral contraceptive
pill containing levonorgestrel 0.15 mg + ethinylestradiol 30 μg per day cyclically (3 weeks treatment—
1 week gap). She developed fever with cough and was diagnosed as a case of pulmonary tuberculosis
after  sputum  smear  examination.  She  was  put  on  isoniazid  (300  mg)  +  rifampin  (600  mg)  +
pyrazinamide  (1.5  g)  +  ethambutol  (1.0  g)  daily  for  2  months,  followed  by  isoniazid  (600  mg)
+  rifampin  (600  mg)  thrice  weekly.  In  the  3rd  month,  she  failed  to  have  the  usual  withdrawal
bleeding during the gap period of contraceptive cycle. After 10 days, her urinary pregnancy test
was  found  to  be  positive.What  could  be  the  reason  for  failure  of  the  oral  contraceptive?
Rifampin  is  known  to  induce  the  metabolism  of  contraceptive  steroids.  Thus,  after  regular intake  of  rifampin  for  more  than  2  weeks  (needed  for  enzyme  induction)  the  steady-state blood level of levonorgestrel and ethinylestradiol could have fallen below the threshold for inhibition of ovulation/contraception. As such, fertility was restored and the woman conceived.
Rifampicin is enzyme inducer and induced metabolism of contraceptives
Ethambutol is enzyme inducer and induce metabolism of contraceptives
Both
None