Dutasteride (Avodart) is a dual 5-alpha-reductase inhibitor — blocks both type 1 and type 2 isoenzymes. Finasteride blocks only type 2.
Mechanism — Reduces serum DHT by ~90% (vs ~70% for finasteride). Slower onset and a five-week elimination half-life make it long-acting in tissue.
Use case — BPH, severe androgenic alopecia. Typical 0.5 mg/day, or off-label lower doses (0.5 mg 2-3x/week). Microdose protocols (topical) reduce systemic exposure.
Caveats — All the post-finasteride syndrome concerns apply more — and stay longer. Side effects can persist for months after discontinuation due to half-life. Not for fertility-active periods. Topical compounds are the lower-risk path.