In this article, we’ll explore the science behind topical finasteride, if incidental exposure to the drug comes with a risk of harming pregnant women and/or children, and strategies you can employ starting today to minimize inadvertently exposing your household to secondary finasteride exposure – all while still preserving your hair gains from the drug.

Topical Finasteride: Benefits & Drawbacks

Finasteride is the world’s most effective FDA-approved treatment for androgenic alopecia in men. Having said that, the drug is not safe for use in children or in pregnant woman – mainly because it inhibits an enzyme that is critical for proper fetal and adolescent development.

Oral formulations of finasteride are ingested by the user, and thereby eliminate almost all possibilities of incidental drug exposure to partners or other household members of the drug user. The only exception to this is a small amount of detectable finasteride in semen, which can be absorbed by a partner during periods of unprotected intercourse – and in amounts that are considered far below danger levels (even while pregnant).^[1]https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2018472/pdf/11785276.pdf

In recent years, interest has exploded in an alternative delivery vehicle for finasteride: topical finasteride. This is because preliminary studies suggest topical finasteride can confer similar hair improvements versus the oral formulations, but also better localize the effects of the drug mainly to the scalp – thus lowering the risk of finasteride’s side effects.

Having said that, the switch to topical formulations comes alongside new risks: incidental drug exposure to partners and other members of the household. But are these risks quantifiable, and if so, how can we reduce them?

How Might Incidental Exposure To Finasteride Occur?

After applying topical finasteride, there is often a period of time – up to 30 minutes – whereby that finasteride solution has yet to fully dry onto the scalp. During this period, it’s much easier for topical finasteride to find its way onto other household surfaces or people – particularly if contact is made with the scalp.

During this window of time (when topical finasteride isn’t yet fully dried), here a few ways inadvertent exposure to topical finasteride can occur:

1. Putting your head down on a pillowcase, thereby rubbing residual finasteride onto sheets that might be shared with your partner and/or children
2. Resting your head on couch pillows and/or furniture surfaces
3. Letting your partner and/or children play with your hair

In all of these scenarios, the amount of exposure to a partner or child will be relatively small. However, with repeated daily exposure throughout critical periods of development – such as a partner’s pregnancy or a child’s adolescence – each repeated contact comes alongside a cumulative increased risk of adverse events.

While the long-term effects of repeated, residual contact by spouses and/or children from topical finasteride have not been thoroughly investigated – and likely never will be (due to the difficulties of conducting such a study) – it goes without saying that it’s within our best interests not to expose our loved ones to unnecessary risks, particularly those that come with the potential for long-long adverse effects.

Topical Finasteride: How To Minimize Incidental Exposure To Loved Ones

Fortunately, when it comes to topical finasteride, there are great science-based management strategies that should allow you to keep using the medication while also minimizing – or even eliminating – your loved ones’ risks of incidental exposure. Here are a few that might take most of the risk off the table.

1. After applying topical finasteride, avoid physical contact for ~30 minutes

Depending on the carrier agents used for topical finasteride (alcohol, propylene glycol, etc.), most topicals should fully dry within a 30-minute window after application. At this point, most of the topical finasteride will remain trapped within the dried portion of the topical.

Once this window is reached, it’s far less likely that scalp contact with pillow cases, furniture, or even a partner’s hands will lead to appreciable leeching off the scalp. So, by making this very simple change, men and women can perhaps take most of the risk away regarding incidental finasteride exposure.

2. Wash pillow cases regularly, and try to dedicate a pillowcase just for the user of finasteride

Even after letting the topical dry, some topical finasteride (albeit significantly less) will leech onto the pillowcase or wherever else you might rest your head. While the amount will be substantially smaller than if you were to rest your hand on that pillowcase before the 30-minute time window for drying is completed, there’s still some topical finasteride that will end up in those sheets.

Under these circumstances, weekly washing of pillowcases is warranted. If that’s not of interest to you or your partner, you can instead dedicate one pillowcase to the person using topical finasteride that the other partner will not use.

If that’s not enough to create peace-of-mind, you can also consider wearing a loosely-fitting shower cap to bed. Some members of our membership community do this – particularly fathers with young boys who don’t want to leave any incidental exposure risks to chance.

3. Time your application windows for when you are out-of-the-house and/or unlikely to have skin contact with household members

Topical finasteride users can dramatically reduce the cumulative exposure risk to loved ones by timing their topical applications to periods where they are (mostly) outside of the home and/or unlikely to come into skin-to-skin contact with other household members. This can be done by either:

1. Applying topical finasteride at least 30 minutes before bed, then washing it out each morning
2. Applying topical finasteride prior to leaving for work, then washing it out after arriving home

If opting for either strategy, it’s critical to let topical finasteride site in the scalp skin for at least 6-12 hours. Otherwise, the diminishment in the drug’s contact time in the scalp may begin to hinder its efficacy. Here’s why.

How Long Should Topical Finasteride Stay on the Scalp Before Washing It Out?

When topicals are applied to the skin, there are multiple stages of absorption.

• Stage #1: the drug sits on the epidermis (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.

So, what’s the minimum amount of time topical finasteride must be left on the scalp to have an effect? First, one needs to know how much of the drug absorbs into the dermis over time.

To answer this, there is an in vitro study measuring percutaneous absorption of topical finasteride across various formulations: ethosomes, ethanol, liposomes, and aqueous (water). ^[2]https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2977015/

Here’s a chart showing the percent of topical finasteride that percutaneously absorbs (enters the dermis) over a 24-hour period:

According to these study results, if applying hydroethanolic finasteride to the scalp, then at the 10-hour mark, ~5 ug/cm² will have been absorbed percutaneously. By hours 22-24, that absorption rises to ~8 ug/cm². That means leaving the drug on the scalp for an additional 12-14 hours before washing it off only offers an additional 3 ug/cm² of percutaneous absorption.

So, say that to minimize a partner’s exposure to finasteride means only keeping it on the scalp for 10 hours before washing it off. This begs the question: is the 5 ug/cm² of absorbed finasteride enough to therapeutically lower scalp levels of dihydrotestosterone (DHT) and regrow hair?

Topical Finasteride: How Long Does It Need To Stay In The Scalp To Be Effective?

There aren’t any clinical studies attempting to answer this question. However, a look at surrogate data can help ballpark an answer.

First, a percentage of topical finasteride will absorb into the dermis, and a smaller percentage of topical finasteride absorbed into the dermis will later absorb into the bloodstream. This means after applying topical finasteride; people can use time-dependent DHT reductions in the bloodstream to “ballpark” how much DHT is likely also getting reduced in the scalp. This is, again, because there is more topical finasteride that percutaneously absorbs than systemically absorbs.

So, 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.

In that regard, see this figure from a 2014 study measuring the effects of one versus two applications of 1 mL of 0.25% topical finasteride on serum DHT levels. ^[3]https://pubmed.ncbi.nlm.nih.gov/25074865

Six hours after applying 1 mL or 2 mL applications of 0.25% topical finasteride (2.275-4.550 mg of finasteride), serum DHT reductions flatline. This means that for the 2 mL application group (the black circle with a dotted line), there was enough systemic absorption by the 6-hour mark to effectively lower serum DHT levels as much as they would normally lower with 1 mg of oral finasteride.

Keep in mind that finasteride has an upper limit for its effects on serum DHT reduction. After hitting ~70% reduction in serum DHT, more finasteride doesn’t reduce serum DHT. The same is true with scalp DHT reductions.

So, the above study shows that with topical finasteride applications of 2.275 mg or higher in a hydroxypropyl chitosan delivery vehicle, systemic reductions in DHT equal that of 1 mg of oral finasteride. And with topical finasteride, since serum DHT reductions are sort of like “canaries in the coal mine” for scalp DHT reduction, we can take an educated guess that 6-12 hours after applying topical finasteride. There’s likely enough percutaneous absorption to therapeutically lower scalp DHT levels for hair regrowth. 

This is corroborated by a 1997 study suggesting that 2 mL daily of 0.005% topical finasteride (0.0912 mg of finasteride exposure) improved hair parameters for men with AGA and without affecting serum DHT levels – even after 16 months of treatment. ^[4]https://www.tandfonline.com/doi/abs/10.3109/09546639709160517

Taken together, all of the above evidence signals that, depending on the exposure level, 6-12 hours of exposure to topical finasteride should be enough to have a therapeutic effect on scalp DHT reductions (and hair growth).

Do these time windows apply to all topical finasteride formulations?

No. Obviously, the contact time required for topical finasteride to be effective depends on a number of factors:

1. The finasteride dilution (i.e., 0.01% vs. 0.1% topical finasteride)
2. The volume of topical applied (i.e., 1 mL vs. 2 mL)
3. The topical’s carrier ingredients (i.e., hydroxypropyl chitosan vs. ethanol vs. propylene glycol vs. liposomes)
4. The frequency of application (i.e., once-daily, twice-daily, every-other-day, etc.)

Nonetheless, a 6-12 hour contact window should allow for therapeutic efficacy of many big-brand topical finasteride formulations available to consumers currently. However, if you’d like to get specific about adjustments on an individual basis, we offer that level of support inside our membership community.

Final Thoughts

For the most part, occasional incidental exposure to topical finasteride should not cause problems for household members. With that said, there are certain groups at an elevated risk – particularly pregnant women and children – and in these groups, cumulative risk over a lifetime of incidental topical finasteride exposure has yet to be studied.

If this is a concern for you, men and women can minimize the risk of incidental drug exposure from topical finasteride by doing any (or all) of the following:

1. Avoiding all physical contact with the scalp for at least 30 minutes after topical finasteride application (so that it fully dries)
2. Dedicating a pillowcase to the topical finasteride user, and washing that pillowcase weekly
3. Wearing a loosely-fitting shower cap to bed
4. Timing topical finasteride applications to 6-12 hour windows where the user is out-of-the-house and/or unlikely to come into contact with household members

Using the evidence and strategies above, our hope is that topical finasteride users can minimize or even eliminate the risk of incidental exposure, all while preserving their hair gains.