Dutasteride is a dual inhibitor of both type I and type II 5-alpha reductase — the enzymes responsible for converting testosterone into dihydrotestosterone (DHT). By inhibiting both isoforms, dutasteride achieves greater DHT suppression than finasteride, which inhibits only type II. This translates to superior clinical efficacy in multiple head-to-head trials for androgenetic alopecia.
Dutasteride suppresses serum DHT by approximately 90% — compared to approximately 70% with finasteride — offering a meaningful pharmacological advantage in hair preservation.
Mechanism of action
DHT is the primary androgen responsible for follicle miniaturisation in androgenetic alopecia. It binds to androgen receptors within the dermal papilla of susceptible follicles, triggering a cascade that progressively shortens the anagen (growth) phase and reduces follicle size. By substantially reducing circulating and intrafollicular DHT, dutasteride interrupts this process — slowing or halting progression and, in many patients, producing measurable regrowth.
Clinical evidence
Multiple randomised controlled trials have demonstrated the superiority of dutasteride 0.5mg over finasteride 1mg for hair count and patient-reported outcomes in male androgenetic alopecia. Evidence for use in female pattern hair loss is also accumulating, particularly for post-menopausal women. Dutasteride is used off-label for hair loss — a prescribing decision made on the basis of an individual patient assessment.
Treatment details
Administration
Oral, daily
Dutasteride is taken orally once daily. Due to its long half-life (~5 weeks), plasma levels are stable and missed doses have minimal short-term impact.
Onset
3–6 months
Meaningful results typically take 3–6 months to become apparent. Maximum benefit is reached at 12–24 months of consistent use.
Commitment
Ongoing
Cessation of dutasteride results in gradual resumption of the hair loss process. Treatment is a long-term commitment rather than a finite course.
Side effects and considerations
Dutasteride is generally well tolerated. The most commonly reported side effects include reduced libido, erectile dysfunction, and reduced ejaculate volume — reported in a minority of users and typically reversible on cessation. A small number of patients report persistent sexual side effects beyond cessation; this is discussed at consultation.
Dutasteride is absolutely contraindicated in pregnancy and in women of childbearing potential due to the risk of feminisation of a male foetus. Female patients must be post-menopausal or using reliable contraception.
PSA (prostate-specific antigen) levels are reduced by approximately 50% with dutasteride use — an important consideration for prostate cancer screening in male patients. This is discussed and documented at consultation.