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Difficulty: HardPsychopharmacology and Psychological Therapies

A 28-year-old woman comes to the clinic for a routine follow-up. She has a 5-year history of bipolar I disorder. Six months ago, she experienced a severe manic episode, and her psychiatric medication regimen was changed due to inadequate symptom control on lithium therapy. At that time, she was transitioned to a different mood stabilizer, and her mood symptoms have remained well-controlled since. She has no other medical conditions. Her only other medication is a combined oral contraceptive pill (ethinyl estradiol and levonorgestrel), which she has taken consistently at the same time every day for the past 3 years without any missed doses. Today, the patient reports a 6-week history of amenorrhea, breast tenderness, and morning nausea. A urine pregnancy test in the clinic is positive. Which of the following psychiatric medications was most likely initiated six months ago?

  1. CarbamazepineAnswer
  2. B
    Lamotrigine
  3. C
    Valproate
  4. D
    Quetiapine
  5. E
    Sertraline

Answer

Carbamazepine
The correct answer is Carbamazepine. This patient presents with an unintended pregnancy despite perfect adherence to her combined oral contraceptive pill (OCP), highly suggesting a pharmacokinetic drug interaction. Combined OCPs are primarily metabolized by the hepatic cytochrome P450 system, specifically CYP3A4. Carbamazepine is a mood stabilizer and anticonvulsant that acts as a potent inducer of several CYP450 enzymes, including CYP3A4. Initiation of carbamazepine significantly increases the metabolism and clearance of ethinyl estradiol and levonorgestrel, leading to subtherapeutic systemic hormone levels and subsequent contraceptive failure. Female patients of childbearing age starting carbamazepine must be counseled to use alternative or backup non-hormonal contraceptive methods (e.g., copper IUD, barrier methods).

Step-by-Step Solution

1
Analyze the patient's clinical presentation and medication history.
The patient has an unintended pregnancy despite perfect adherence to combined oral contraceptives, following the recent initiation of a new mood stabilizer for bipolar I disorder.
Identifying contraceptive failure in a highly adherent patient points toward a pharmacokinetic drug-drug interaction.
2
Determine the required mechanism of the drug interaction.
The new medication must be a strong inducer of hepatic cytochrome P450 enzymes (specifically CYP3A4), which metabolize the estrogen and progestin components of the contraceptive.
Enzyme induction accelerates the clearance of the contraceptive hormones, causing subtherapeutic systemic levels and allowing ovulation to occur.
3
Identify the medication from the options that matches this pharmacokinetic profile.
Carbamazepine is a well-known, potent CYP3A4 inducer used in the treatment of bipolar disorder.
Other options either do not affect CYP3A4, act as inhibitors, or have their own metabolism induced by contraceptives without affecting the contraceptive itself.

Key Concept

Carbamazepine is a potent inducer of hepatic cytochrome P450 enzymes (particularly CYP3A4). It accelerates the metabolism of combined oral contraceptives, which can lead to subtherapeutic hormone levels and unintended pregnancy.
Estimated Time:1m 15s
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Topics

Psychopharmacology and Psychological Therapies

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