Topical Finasteride Absorption into the Bloodstream
Yes, topical finasteride is absorbed into the bloodstream, but at significantly lower levels compared to oral finasteride, with plasma concentrations approximately 100 times lower than oral administration.
Pharmacokinetics of Topical Finasteride
Topical finasteride formulations are designed to deliver the medication directly to hair follicles while minimizing systemic exposure. However, research confirms that some systemic absorption does occur:
- In a phase III randomized controlled trial, maximum plasma finasteride concentrations were more than 100 times lower with topical formulations compared to oral finasteride 1
- Topical finasteride reduces serum DHT levels by approximately 34.5% compared to 55.6% reduction seen with oral finasteride, indicating systemic absorption but with less systemic impact 1
- A 7-day pharmacodynamic study showed that twice-daily application of 0.25% topical finasteride solution provided similar inhibition of plasma DHT as once-daily oral finasteride 1mg, confirming systemic absorption 2
Comparison with Oral Finasteride Absorption
The FDA label for oral finasteride provides important context for understanding systemic absorption:
- Oral finasteride has a bioavailability of approximately 63% (range 34-108%) 3
- Maximum plasma concentration after oral administration averages 37 ng/mL and is reached within 1-2 hours 3
- Approximately 90% of circulating finasteride is bound to plasma proteins 3
- The mean elimination half-life in plasma is 6 hours (range 3-16 hours) 3
Clinical Implications of Systemic Absorption
The reduced systemic absorption of topical finasteride has important clinical implications:
- Lower systemic exposure means reduced risk of sexual dysfunction side effects commonly associated with oral finasteride 2
- Side effects of topical formulations tend to be localized to the application site (scalp pruritus, burning sensation, irritation, contact dermatitis, and erythema) 2
- Despite lower systemic absorption, topical finasteride demonstrates similar efficacy to oral finasteride in treating androgenetic alopecia 1
- In clinical trials, topical finasteride significantly improved hair count compared to placebo (adjusted mean change 20.2 vs. 6.7 hairs) and was numerically similar to oral finasteride 1
Factors Affecting Absorption
Several factors can influence the degree of systemic absorption of topical finasteride:
- Formulation type (solutions, gels, liposomes, etc.) affects penetration rates
- Liposomal gel systems containing 2% methyl cellulose and gel systems with poloxamer P407 exhibited the highest flux values (28.4 ± 1.3 μg/cm²h and 23.1 ± 1.4 μg/cm²h, respectively) 4
- Penetration enhancers like propylene glycol, commonly used in topical finasteride formulations, can increase absorption 4
- Application site condition (intact vs. damaged skin) may affect absorption rates
- Frequency and duration of application influence cumulative systemic exposure
Clinical Considerations
When considering topical finasteride use:
- Patients concerned about systemic side effects of oral finasteride may prefer topical formulations due to lower systemic exposure 2
- Despite lower systemic absorption, patients should still be informed about potential systemic effects
- Topical finasteride may be particularly beneficial for patients who experienced adverse effects with oral finasteride but still require treatment
- The similar efficacy with reduced systemic exposure makes topical finasteride an attractive alternative to oral administration 1
Conclusion
While topical finasteride was developed to minimize systemic exposure by targeting hair follicles directly, research clearly demonstrates that it is absorbed into the bloodstream, albeit at significantly lower concentrations than oral finasteride. This reduced systemic exposure translates to a lower risk of systemic side effects while maintaining similar efficacy in treating androgenetic alopecia.