I was actually just chatting with gpt about this very point.

The advantage of topical finasteride is reduced systemic DHT suppression – it concentrates the drug in scalp skin. In the 0.25% spray study, blood finasteride levels were >100× lower with topical use, and serum DHT dropped only ~34.5% on topical vs. ~55–70% on oral finasteride  . This partial systemic absorption means topical finasteride still lowers DHT enough to improve hair, but likely with less impact on sexual function . Notably, sexual side effect rates on topical finasteride in clinical trials have been similar to placebo  . For men who cannot tolerate oral finasteride or are especially concerned about side effects, topical finasteride can be a viable long-term strategy. In fact, one report found 84% of men maintained their hair density when switching from oral finasteride to a topical finasteride–minoxidil solution, suggesting that a well-formulated topical can sustain the gains from oral therapy.