A woman with known etonogestrel implant (Nexplanon) in place for 2 years presents with an unintended pregnancy. Which mechanism most likely explains the failure?

• A Subdermal implant placed in adipose tissue causing reduced etonogestrel absorption
• B Implant expiration (approved for only 1.5 years)
• C Development of anti-progestogen antibodies
• D Concurrent use of enzyme-inducing drugs (e.g., rifampicin) accelerating etonogestrel metabolism ✓

Explanation

Etonogestrel subdermal implant (Nexplanon) is approved for 3 years and has one of the lowest failure rates of any contraceptive (<0.1 per 100 woman-years). The primary mechanism of clinical failure is concurrent use of enzyme-inducing medications — including rifampicin, rifabutin, phenytoin, carbamazepine, and efavirenz — which induce CYP3A4 and accelerate etonogestrel metabolism, dropping serum levels below the threshold for ovulation suppression. In such cases, additional contraception is advised.

Reference: Shaw's Textbook of Gynaecology, 17th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.