Here’s an excerpt from a post I made several months back on the subject of finasteride.
There are a few studies (8 to be specific) that show that topical use of finasteride brings a higher concentration of the drug to the scalp where you need it and with less systemic absorption. The studies are limited in that they were fairly small studies and there is much heterogeneity in the methods, but they all generally showed positive hair growth with topical use. In one of these studies, they actually measured DHT levels while administering the drug topically and orally and compared these to baseline (no-finasteride use). They demonstrated that approximately 20% of the topically applied drug is absorbed systemically.
Here’s a link to a fairly recent (2018) systematic review on the subject. You can download a free copy of the paper from PubMed. Keep in mind that there are only 8 studies with sufficient data to be included in the review and they were all fairly small studies with limitations.
Here are some notes that I made after reading the review and the studies that I could download from PubMed.
Review article of 8 studies known to exist.
• Most common dose was 1 mL of 0.25% (2.3 mg/mL) twice daily = 4.6 mg finasteride per day.
• In all studies, there was significant decrease in the rate of hair loss, increase in total and terminal hair counts, and positive hair growth assessment with topical FNS.
• Most relevant study was a study of a 23 men because they compared DHT blood levels of 4.6 mg topical vs. 1 mg oral.
○ Plasma DHT was reduced by 68–75% with use of topical FNS
○ Plasma DHT was reduced by 62–72% with oral FNS
○ This indicates that there is approximately 1/5 the systemic absorption with topical vs. systemic.
○ Caserini M1, Radicioni M, Leuratti C, et al. A novel finasteride 0.25% topical solution for androgenetic alopecia: pharmacokinetics and effects on plasma androgen levels in healthy male volunteers. Int J Clin Pharmacol Ther. 2014;52(10):842–9. [PubMed: 25074865]