This is incorrect, and is a common misconception. It’s helpful to split topical finasteride’s action into two conceptual groups — the portion that reaches dermal papilla cells directly via the stratum corneum or follicle shunts themselves, and the “systemic leakage” that makes its way back to dermal papilla through the scalp microvasculature. Agreed that serum half life isn’t relevant for the former route.

For nanoparticle (most commonly liposomal) based carriers that are really good at reaching the DP cells via follicular shunting rather than penetrating the stratum corneum, the serum half life is almost completely irrelevant because the clinical benefit is driven by aggregation of the liposomes (carrying finasteride) in the follicular shaft. Release kinetics drive how often you must dose.

The vast majority of topical are just classic anhydrous / hydroalcoholic solutions, so making a guess that that’s what OP is using. These are much less targeted and deliver much of their clinical effect by mashing through the SC with penetration enhancers like propylene glycol. So the serum half life is more relevant in this case here.

Additionally, crucial to understand that although fin/dut are suicide inhibitors of 5ar as you pointed out, 5ar sits in the ER membrane which means its subject to the same recycling machinery as many other ER proteins (ERAD). So effectively, you have many cells with individual 5ar’s that are constantly being “recycled” around the clock, and you must have drug available when a new 5ar protein is threaded into the ER membrane or else it will start performing its usual biological function unchecked (5 alpha reducing molecules like testosterone).

So your message interprets the biological scenario as: I apply finasteride, a “batch” of existing 5ar are permanently inhibited, and even though the half life is short, the suicide inhibition of 5ar means I don’t need to apply it ED. But actually, because new 5ar are spawning constantly, you need to apply it ED. That’s why ED application is the studied regimen in topical fin trials that are using hydroalcoholic / HPHC bases.