Finasteride is a hair loss drug for treatment of androgenic alopecia. It inhibits the 5-alpha reductase type II enzyme that converts testosterone into dihydrotestosterone, or DHT.
Topical finasteride formulations (think: solution, spray, or gel) are growing in popularity because of their localized approach. Oral finasteride reduces DHT everywhere in the body. Topical finasteride reduces DHT in the area that it is applied while minimizing systemic absorption. That is, if you apply it correctly. The question of how to apply topical finasteride for the greatest benefit and the least systemic absorption has no single, easy answer.
Applying Topical Finasteride to the Scalp
Strategies for applying topical finasteride depend on its formulation (spray, solution, serum, or gel). We’ll review those first, then explain how long topical finasteride needs to stay on the scalp in order to absorb and have an effect (hint: it depends on the dosage and carrier agent).
How to Apply Topical Finasteride Spray
Many hair loss sufferers opt for topical finasteride spray. These sprays are often used to disperse the drug over wide surface areas of scalp skin, and are most appropriate for people with (1) thinning over widespread areas who also (2) are keeping their hair very short so that the scalp skin is easily accessible.
If you have diffuse thinning and longer hair, sprays might not be right for you. This is because too much of the topical finasteride will end up on the hair (where it does nothing) rather than the scalp (where hair grows from).
Users are advised to hold the spray bottle 2-3 inches from the scalp and apply the prescribed amount on the crown, top, and front of their heads. The hands can then be used to rub in any liquid blocked by existing hairs. This will ensure that a considerable amount of the formula has reached the scalp, particularly in problem areas.
Some users prefer to spray the formula into their palms and apply it manually with their fingertips. As with other formulas, parting sections of hair may make it easier to apply.
How to Apply Topical Finasteride Solution
Topical finasteride is also prescribed in the form of a liquid solution. These variations may include minoxidil as a combination therapy. Such formulas typically include a dropper for easy application. To apply the solution, part the hair in sections, apply a few drops along the part, then rub the solution around the scalp skin with the fingertips. Additional drops can then be applied to key problem areas (crown, top, and both sides of the front).
Alternatively, the prescribed amount can be dispensed into the hand and applied manually.
How to Apply Topical Finasteride Gel
Gel formulations are often made of bases of silica and/or liposomes. They’re very popular, as they are easy to apply. For best results, users can start with a few pumps of the gel, applying with their hands to provide even coverage across each problem area. The gel should gently be massaged into the scalp for 30 seconds or more.
No matter the treatment form, it’s only necessary to cover the areas affected by hair loss. Users should always follow their doctor’s recommendations and consult the directions included in their formula.
Any excess formula should immediately be washed off of hands and other areas of the body.
How Long Should Topical Finasteride be left on Scalp?
How long to let topical finasteride sit on your scalp depends on several factors, including:
- The dilution (the % finasteride in your topical formulation)
- The daily mL applied (alongside % dilution, this will determine your daily total exposure volume of finasteride in mg)
- The carrier ingredients in your topical (this will determine how much – and how quickly – topical finasteride will move from outside of the scalp into the dermis, where it can actually elicit an effect on hair follicles)
To understand more of this process, we also need to understand the stages of absorption when applying any drug to our scalp skin:
- Stage #1: the drug sits on the epidermis (i.e., the outermost layer of skin). Over several hours, some of the drug will absorb into the dermis, and some will evaporate.
- Stage #2: the drug absorbs percutaneously. This is when some of the drug moves from the epidermis into the dermis, where it can begin to affect hair follicles.
- Stage #3: the drug absorbs systemically. This is when some of the drug moves from the dermis into the bloodstream, where it can have systemic effects.
An in vitro study measured percutaneous absorption of topical finasteride across a variety of different formulations: ethosomes, ethanol, liposomes, and aqueous (water). ncbi.nlm.nih.gov/pmc/articles/PMC2977015
This chart shows the percent of topical finasteride that percutaneously absorbs (enters the dermis) over a 24-hour period:
There are three big takeaways from this chart:
- The longer topical finasteride is left on the scalp, the more drug will be absorbed percutaneously.
- Across carrier ingredients, absorption rates appear to be linear. At 10 hours of contact, 2-13 micrograms of finasteride are absorbed into the skin, depending on the carrier. At 20 hours, those numbers double to 4-26 micrograms.
- When it comes to finasteride skin penetration, carriers rank from best to worst as ethosomes > hydroethanol > lipsomes > water.
Finasteride Absorption Rates
Leaving finasteride on the scalp for 10 hours allows for 5 micrograms per square centimeter of percutaneous absorption across most carrier agents. Is this enough finasteride to therapeutically lower scalp DHT levels? More importantly, is it enough to regrow hair?
There is not a clinical study that attempts to answer this. However, there is some surrogate data to help us approximate the answer.
A certain percentage of topical finasteride will absorb into the dermis. The bloodstream will absorb some of this later. Time-dependent DHT reductions in the bloodstream can be used to “ballpark” how much DHT is likely also being reduced in the scalp. This is because there is more topical finasteride that percutaneously absorbs than systemically absorbs. For lower dosages, the effects we see in the system can be used as signals for what’s happening – at a minimum – in the scalp skin.
This figure from a 2014 study measures the effects of one versus two applications of 1 mL of 0.25% topical finasteride on serum DHT levels. https://pubmed.ncbi.nlm.nih.gov/25074865/
Optimal Amount of Time
Six hours after applying 1 mL or 2 mL applications of 0.25% topical finasteride (i.e., 2.275-4.550 mg of finasteride), serum DHT reductions flatline. By the 6-hour mark, the 2ml application group experienced roughly the same serum DHT reductions as would be expected from a 1mg oral dose of finasteride.
Finasteride has an upper limit for its effects on serum DHT reduction. After ~70% reduction in serum DHT, adding more finasteride doesn’t reduce more serum DHT. The same is true with scalp DHT reductions.
Topical finasteride applications of 2.275 mg or higher (with a hydroxypropyl chitosan delivery vehicle) achieve systemic reductions in DHT. Furthermore, the larger dose of topical finasteride reduces systemic DHT levels on par with oral finasteride.
Serum DHT reductions are only achieved after scalp DHT reductions occur. Because of this, the conclusion can be drawn that 6-12 hours after applying topical finasteride, there’s likely enough percutaneous absorption to therapeutically lower scalp DHT levels for hair regrowth. However, this time window depends on a number of factors, including:
- Finasteride dilution (%). After all, the percent of finasteride, in part, determines your total daily exposure of finasteride. Remember that finasteride’s absorption is linear, so the higher the dilution, the more finasteride will absorb.
- The daily mL applied. Along with the percentage dilution of finasteride, the mL of topical applied will determine our total daily exposure of finasteride placed on the scalp.
- The carrier ingredients used. As we learned in the previous study, water-based carriers perform the worst at carrying finasteride into the skin, while ethosomes and water-alcohol mixes perform the best.
The next logical question becomes, just how low of a dilution of finasteride can you apply to evoke hair growth outcomes, while still potentially preserving serum DHT levels to the best of your ability? Interestingly, there seems to be a “sweet spot” for this at ~0.1 mg daily of topical finasteride mixed with alcohol and propylene glycol as carriers.
A 1997 study corroborates this. The study suggests that 2 mL daily of 0.005% topical finasteride (i.e., 0.0912 mg of finasteride exposure) improved hair parameters for men with AGA. This was achieved without affecting serum DHT levels – even after 16 months of treatment. https://www.tandfonline.com/doi/abs/10.3109/09546639709160517
For Most Topical Finasteride Formulations, 6-12 Hours is Long Enough
Most big-brand topical finasteride companies are:
- Selling dilutions mixed with alcohol, propylene glycol, or glycerin
- Selling dilutions of 0.1% to 0.3% finasteride
- Advising patients to apply at least 1-2 mL daily
For most users, this will equate to 1 mg to 6 mg of topical finasteride exposure daily – which is more than 100x the minimum viable dose of topical finasteride. At these dosage ranges, the evidence suggests that topical finasteride will only need 6-12 hours on the scalp to therapeutically lower scalp DHT levels and start encouraging hair growth.
Topical finasteride users should use this time window to their advantage! But they should also keep in mind that, at most big-brand dose ranges, they’re exposing themselves to just as much (if not more) finasteride than the oral formulations. So they might be overpaying for topical finasteride in hopes of “localizing” its effects, only to also have just as much – if not more – of that drug going systemic.
To get the true benefits of localization, users will likely need to drop their dose as low as 0.005% x 2 mL daily, and periodically track serum DHT levels to ensure systemic effects are minimal.
The instructions on the packaging aren’t always as precise as the research. Best to trust the science when it comes to how to apply topical finasteride for maximum effectiveness, and minimal side effects.
Rob English is a researcher, medical editor, and the founder of perfecthairhealth.com. He acts as a peer reviewer for scholarly journals and has published five peer-reviewed papers on androgenic alopecia. He writes regularly about the science behind hair loss (and hair growth). Feel free to browse his long-form articles and publications throughout this site.