How does finasteride work and why avoid in pregnancy?

• A. Inhibits 5-alpha-reductase type II; teratogenic to male fetuses; avoid in pregnant women
• B. Inhibits aromatase; safe in pregnancy
• C. Increases testosterone conversion to estrogen
• D. Enhances DHT production

Answer: (A)

Finasteride works by blocking the enzyme 5-alpha-reductase, primarily the Type II isoenzyme. This enzyme normally converts testosterone into dihydrotestosterone (DHT), a potent androgen that drives growth in hair follicles and the prostate. By inhibiting this conversion, finasteride lowers DHT levels, which helps slow hair follicle miniaturization in androgenetic alopecia and reduce prostate enlargement.

Pregnancy precaution makes this drug unique: DHT signaling is essential for male fetal genital development. If a pregnant woman is exposed to finasteride, especially during early pregnancy, it can interfere with the masculinization process of a male fetus, potentially leading to undervirilization or ambiguous genitalia. Therefore, finasteride is contraindicated in pregnancy, and handling broken or crushed tablets by women who may become pregnant is also avoided.

The other options don’t fit because they describe different enzymes or effects (aromatase inhibition, increasing estrogen formation, or increasing DHT production), none of which reflect finasteride’s actual mechanism or its pregnancy risk.