Half-life (drug half-life)
A medicine's half-life is the time it takes for the amount of the drug in the blood to fall by half, which shapes how long the medicine stays in the body after the last dose.
Full definition
If a medicine has a half-life of six hours, then six hours after a dose about half of it is left in the blood, six hours later about a quarter, and so on. The figure for each licensed medicine is in section 5.2 of its SmPC, and the three oral hair loss medicines differ widely. For finasteride 1 mg, the Propecia SmPC gives a mean terminal half-life of about 5 to 6 hours in men aged 18 to 60, and about 8 hours in men over 70. For dutasteride 0.5 mg, the Avodart SmPC says that with daily dosing "the half-life is approx. 3-5 weeks", so it takes months rather than days to leave the body. The same SmPC notes that if another medicine slows dutasteride's breakdown, the long half-life may be prolonged further; see dutasteride and CYP3A4 inhibitors. For oral minoxidil, the Loniten SmPC gives an average plasma half-life of about 4 hours, but adds that its effect lasts several days. Half-life matters in practice when you stop a medicine, when you start another one that might interact, and for rules that depend on a drug having cleared, such as blood donation. Our safe discontinuation guides for finasteride, dutasteride and oral minoxidil explain what this means for each one. Both finasteride and dutasteride carry MHRA warnings: finasteride is associated with depression, suicidal thoughts and sexual side effects that may persist after stopping, and dutasteride carries a precautionary mood warning. With dutasteride's long half-life, anything you notice will also take longer to clear after stopping.
What it does not mean
Half-life is about the drug in your blood, not about how long its effects last. A short half-life does not mean the benefit or side effects vanish within hours: the Propecia SmPC says that after stopping finasteride the beneficial effects on hair begin to reverse by six months and return to baseline by 9 to 12 months, and the Loniten SmPC says minoxidil acts for several days despite a 4-hour half-life. The MHRA also recognises that sexual side effects of finasteride may persist after treatment has stopped, which is not explained by how quickly the drug itself is cleared.
UK regulatory source
Propecia 1 mg SmPC sections 4.2 and 5.2; Avodart 0.5 mg SmPC sections 4.5 and 5.2; Loniten 2.5 mg SmPC section 5.2 (all eMC, MHRA-approved product information); MHRA Drug Safety Updates, 29 April 2024 and 11 May 2026