Dutasteride

DHT is the high-potency androgen that drives prostate enlargement and, in genetically susceptible men, the follicle miniaturization of male-pattern hair loss. It is made from testosterone by the enzyme 5-alpha-reductase, which comes in two main isoenzymes, type 1 and type 2. Dutasteride is a synthetic 4-azasteroid that binds and inhibits both isoenzymes essentially irreversibly, which is the key structural difference from finasteride; it is approved as a once-daily 0.5 mg capsule (Avodart) to shrink the prostate, ease urinary symptoms, and lower the risk of acute urinary retention and BPH surgery ^[1].

Why does hitting both enzymes matter? Type 2 predominates in the prostate, which is why finasteride works there, but type 1 is important in skin, scalp, and liver, and both isoenzymes turn out to be active in benign prostate tissue. Blocking only type 2 leaves type 1 free to keep making DHT, so finasteride suppresses circulating DHT by roughly two-thirds, whereas dual inhibition with dutasteride produces a greater and more rapid drop, commonly cited around 90% or more ^[2][8]. That difference is not just academic: type 1 expression rises in prostatic intraepithelial neoplasia and prostate cancer, and both isoenzymes appear elevated in higher-grade disease, which is the rationale for expecting dual inhibition to matter in the prostate ^[3][4].

For hair, the evidence is genuinely favorable but mostly short-term. In a randomized dose-ranging study, dutasteride raised scalp hair count in a dose-dependent way and the 2.5 mg dose beat finasteride 5 mg at 12 and 24 weeks, alongside larger reductions in scalp and serum DHT ^[5]. A later phase III trial in over 900 men found dutasteride 0.5 mg significantly better than both finasteride 1 mg and placebo on hair count and photographic assessment at 24 weeks, with a similar adverse-event profile ^[6]. Reviews and meta-analyses reach the same broad conclusion, that dutasteride tends to edge out finasteride for AGA with comparable tolerability, which is part of why it earned an alopecia approval in South Korea and Japan even though it remains off-label elsewhere ^[7].

In BPH the case is well established: pooled two-year placebo-controlled phase III studies showed dutasteride improved symptoms and flow, cut prostate volume, and reduced acute urinary retention and surgery, with benefits durable out to four years ^[8][9]. The CombAT trial showed that combining dutasteride with the alpha-blocker tamsulosin beat either drug alone for symptom relief and progression ^[10]. The prostate-cancer story is the nuanced one. The four-year REDUCE trial found dutasteride cut biopsy-detected prostate cancer by about 23% versus placebo, but during years three and four there were more Gleason 8 to 10 tumors in the dutasteride arm (12 versus 1), the same kind of high-grade signal that dogged finasteride's PCPT, and its meaning is still argued ^[11][12]. A REDUCE analysis also found heavy alcohol intake erased the protective effect and raised high-grade risk, a reminder the picture is not simple ^[13]. Because 5-alpha-reductase inhibitors roughly halve PSA, any PSA reading in a man on dutasteride has to be doubled to stay meaningful.