Paper II
2019 July (Supplementary) (2010 Scheme)

Question

Mrs. Mala aged 30 is a known epileptic was on treatment with Tab. Phenytoin for the past 3 yrs. She was taking oral contraceptive pills for the last 6 months. She did not get her menstrual cycle for the past 2 months. On investigation, her pregnancy was confirmed  What is the therapeutic problem  Why she became pregnant in spite of taking oral contraceptive pills  Name 2 other drugs which will have problems with phenytoin.  Add a note on emergency contraception. (1+2+1+2)

Q26 marksEssays

Answer

Therapeutic problem

  • Contraceptive failure due to a drug interaction between phenytoin and the oral contraceptive pill, resulting in an unplanned/unwanted pregnancy in a woman on a teratogenic antiepileptic drug

Why she became pregnant despite OCPs

  • Phenytoin is a potent hepatic microsomal (CYP450) enzyme inducer
  • It significantly increases the metabolism of the oestrogen and progestin components of the oral contraceptive pill, lowering their plasma levels below the threshold needed for reliable contraceptive efficacy — leading to breakthrough ovulation and contraceptive failure

Two other drugs with interaction problems with phenytoin

  • Warfarin (phenytoin alters its metabolism/protein binding — unpredictable anticoagulant effect)
  • Corticosteroids (phenytoin reduces their efficacy via enzyme induction)

Note on emergency contraception

  • Used after unprotected intercourse to prevent unwanted pregnancy
  • Levonorgestrel (single high dose within 72 hours) — most commonly used, though efficacy may be reduced in patients on enzyme-inducing drugs like phenytoin (higher dose or alternative method needed)
  • Ulipristal acetate — effective up to 120 hours
  • Copper IUD — most effective, unaffected by enzyme induction, can be inserted up to 5 days after intercourse
  • Mechanism: primarily inhibits/delays ovulation; not effective once implantation has occurred
  • Given the enzyme-induction interaction established in this case, the copper IUD is the most reliable emergency contraceptive option for a patient on phenytoin, since hormonal methods are also subject to reduced efficacy from the same enzyme induction

Other Years Asked


Revise MBBS
Preview