Finasteride primarily inhibits which 5-alpha-reductase isoenzyme, and what is a key safety consideration?

• A. Type II; pregnancy precautions apply because of fetal risk.
• B. Type I; very few safety considerations.
• C. Both Type I and II; no specific precautions.
• D. Type II; used for male pattern hair loss; pregnancy precautions apply.

Answer: (D)

Finasteride blocks the enzyme that converts testosterone to dihydrotestosterone (DHT) and does so mainly by inhibiting the type II isoenzyme of 5-alpha-reductase. This type II enzyme is the predominant source of DHT in the hair follicles and prostate, so lowering its activity decreases DHT levels in those tissues. That reduction helps treat male pattern hair loss and also can ease BPH symptoms, which is why finasteride is used for hair loss in men.

A crucial safety consideration is pregnancy risk. DHT is important for fetal male development, and exposure to finasteride can cause feminization of a male fetus. Therefore, pregnancy precautions apply: pregnant women should not handle broken or crushed tablets, and the drug is avoided during pregnancy. In short, finasteride is a type II-selective 5-alpha-reductase inhibitor used for male pattern hair loss, with important pregnancy-related precautions.