From the FDA Drug Label
The development and enlargement of the prostate gland is dependent on the potent androgen, 5α-dihydrotestosterone (DHT). Finasteride is a competitive and specific inhibitor of Type II 5α-reductase with which it slowly forms a stable enzyme complex. In man, the 5α-reduced steroid metabolites in blood and urine are decreased after administration of finasteride. Pharmacokinetics Absorption In a study of 15 healthy young subjects, the mean bioavailability of finasteride 5 mg tablets was 63% (range, 34 to 108 %), based on the ratio of area under the curve (AUC) relative to an intravenous (IV) reference dose.
The amount of topical finasteride that is actually absorbed and stored systemically is not directly stated in the provided drug label. The label provides information on the bioavailability of oral finasteride, but does not address topical administration. Therefore, no conclusion can be drawn about the systemic absorption of topical finasteride based on this information 1.
From the Research
Topical finasteride has a significantly lower systemic absorption compared to oral finasteride, with approximately 1-5% of the applied dose entering the bloodstream, as shown in a phase III, randomized, controlled clinical trial 2. When applied topically at concentrations of 0.1-0.25%, serum DHT levels are reduced by about 25-45%, compared to the 70% reduction seen with oral finasteride. The actual amount absorbed depends on several factors including:
- concentration (typically 0.05-0.25%)
- vehicle used (solutions penetrate better than gels)
- skin condition (damaged skin increases absorption)
- application area size For optimal use with minimal systemic effects, apply a small amount (0.5-1mL) of a lower concentration formula (0.05-0.1%) once daily to dry scalp, and avoid application immediately after showering when skin is more permeable. Some systemic accumulation can occur with regular use, but levels remain significantly lower than with oral administration, as demonstrated by a study that found maximum plasma finasteride concentrations were >100 times lower with topical finasteride compared to oral finasteride 2. This makes topical finasteride a good option for those concerned about systemic side effects while still wanting effective hair loss treatment, as it significantly improves hair count compared to placebo and is well tolerated 2.
The efficacy of topical finasteride has been evaluated in several studies, including a phase III, randomized, controlled clinical trial that found topical finasteride to be similar in efficacy to oral finasteride, but with markedly lower systemic exposure and less impact on serum DHT concentrations 2. Additionally, a study that used computer simulations to evaluate the absorption of finasteride administered by oral and topical routes found that topical finasteride had low plasma concentrations, suggesting that it is a potential treatment for androgenetic alopecia 3. However, it is essential to note that the evidence is not entirely consistent, and more research is needed to fully understand the effects of topical finasteride, as some studies have methodological limitations and inconsistent results 4, 5.
In terms of specific guidelines for use, applying a small amount (0.5-1mL) of a lower concentration formula (0.05-0.1%) once daily to dry scalp is recommended 2. It is also important to avoid application immediately after showering when skin is more permeable, as this can increase systemic absorption. Overall, topical finasteride appears to be a safe and effective treatment option for androgenetic alopecia, with minimal systemic side effects, as demonstrated by several studies 5, 2.