Hair guideHair-Loss Glossary5-alpha-reductase

5-alpha-reductase

Last updated: 2026-06-14

The enzyme that converts testosterone into DHT. There are two main types; finasteride blocks type II, while dutasteride blocks both. Inhibiting this enzyme is the mechanism behind the most effective anti-DHT hair-loss drugs.

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Sources: AAD ↗

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Not medical advice. General education only; it does not replace diagnosis or treatment by a licensed professional. Consult a board-certified dermatologist before starting, stopping or changing any treatment.

⚠️ When to see a doctor — don’t self-treat

  • Sudden patchy or circular bald spots
  • Redness, scaling, pus, pain or itch (possible scarring alopecia — treat urgently)
  • Broken hairs or rapid loss
  • Loss with body-wide signs (weight loss, fatigue, cycle changes, acne, extra hair)
  • Loss right after a new medication
  • Any hair loss in a child
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