Science Explained: How Do Lash Growth Serums Work?

It’s 2021. Hot girl summer is in full effect. You’re vaccinated. You broke out all the swimsuits you bought last year as well as some new ones. You want to look your best at the *insert body of water here*. Unfortunately, waterproof mascara is a pain in the ass, and no one likes scrubbing their eyes for 20 minutes after wearing it only to still wake up looking like a raccoon. Lash serums are very tempting.

lash growth serums not needed
This is my friend Josie’s eye. It’s been featured on BuzzFeed and went semi viral. Her lashes are phenomenal. If your lashes don’t naturally look this gorg, I’m here for you with the science you need to know before you start trying things.

Today I’m explaining the science behind a few common questions: How do lash growth serums work? Are they safe? What’s the difference between Latisse and over-the-counter lash serums?

How lash growth serums were discovered

Latisse is basically the accidental discovery of a side-effect. Bimatoprost is a medication that was initially used to treat glaucoma, a condition in which the optic nerve is damaged, often due to increased pressure in the eye. Bimatoprost works by reducing this pressure. It was first approved for glaucoma treatment in 2001, and after realizing the medication’s potential to cause lash growth, it underwent a separate FDA approval under the name Latisse in 2008. The concentration (ratio of active ingredient to other ingredients) is the same for both, at 0.03%.

How prescription lash growth serums work

Bimatoprost is a prostaglandin analog, meaning that it is similar to and works on the same cellular receptors as prostaglandins, a category of compounds that are naturally found all over the human body.

A point I think is worth mentioning because I keep seeing it pop up on other sites discussing lash serums — prostaglandin analogs are not hormones. Prostaglandins aren’t technically hormones either. This is because they don’t travel through the body to interact with the tissue they impact. “Hormone-free” lash serums don’t mean prostaglandin analog-free.

When prostaglandin receptors in hair follicles are activated by prostaglandin analogs, they move from a resting phase to a growth phase. As a result, lashes that were “resting” begin to grow again. What’s more, is that prostaglandin analogs can also induce hypertrophy of the follicle — meaning the follicle grows, and the hair gets thicker.

This explains the reason Latisse works to grow lashes thicker and longer, but why does lash serum make lashes darker? It all has to do with melanin production. In our bodies, melanocytes (skin cells) contain melanosomes, the part of the cell responsible for creating the pigment melanin. In those who have been treated with bimatoprost, melanin production is increased, and lashes get darker.

While more melanin (pigment) is produced by these melanosomes, there isn’t evidence that shows more melanocytes are produced. This is good news, as if there were significant increases in the number of melanocytes, this could pose a problem since (oversimplifying here), unregulated cell division poses a cancer risk.

What’s the difference between Latisse and over-the-counter lash serums?

Over-the-counter lash growth serums contain a variety of products. Many of them contain moisturizing ingredients like vitamin E oil or hyaluronic acid and vitamins like biotin. It is important to note that Latisse is the only FDA-approved lash growth serum and the only one that contains the active ingredient bimatoprost.

However, bimatoprost isn’t the only prostaglandin analogue in many of these lash serums. Isopropyl cloprostenate is fairly common, as well as Prostaglandin I.C.

Many of the “natural” lash serums on the market have a variety of plant extracts and other vitamins in them. There’s pretty sparse research on this, so further research could be done into these compounds that occur naturally in plants to see if they act as prostaglandin analogs and work the same way. Alternatively, some plant extract mixtures seem to activate various growth factors that play a role in hair follicle growth through a different process than prostaglandin analogs.

Legally speaking, over-the-counter lash growth serums are not supposed to make claims that they can “grow” or “increase”. This is in violation of the Federal Food, Drug, and Cosmetic Act, which states that an active ingredient intended to affect the structure or function of the body should be classified as a drug.

But just because companies shouldn’t be doing this, doesn’t mean that they don’t. A quick Google shows that plenty of companies advertise lash growth serums’ potential to alter the actual structure of the lashes. While some have wised up since RapidLash got hit with an FDA warning letter in 2011, some haven’t gotten the memo.

Are lash growth serums safe?

I realize I have a tendency to write these articles, list a bunch of potential side effects, and then ultimately say that something doesn’t work and is probably a waste of your money. Overall, Latisse and other prostaglandin-analog-based lash growth serums do work to make your lashes grow. With over-the-counter lash growth serums, it’s a little harder to say.

Most (if not all) of the active ingredients in over-the-counter lash growth serums have not been clinically tested to determine lash growth. Cosmetics are regulated differently than drugs in the US, so there’s no need for a company to prove that something works and there’s no need to demonstrate the safety of ingredients before a cosmetic is sold. This means that they also don’t have to make side effects known to consumers.

Whether not over-the-counter lash growth serums are “safe” is kind of up to interpretation. Are you allergic to any of the ingredients in the lash serum? Are any of the ingredients common eye/skin irritants? If not, it’s unlikely that there is any long-term harm caused by using them. They likely just have varying degrees of effectiveness and lack data to back up their claims. They may work, they may not.

But for OTC prostaglandin-analog-containing lash serums, it’s a bit of a different story. Where over-the-counter lash growth serums differ from Latisse is in that these serums don’t show how much prostaglandin analog is contained in the product (meaning that the percentage of active ingredient can vary widely), AND they often have many of the oils and essences in the other OTC serums.

Side effects of prostaglandin analogs in lash growth serums

While the side effects of many OTC lash serums are kind of up in the air, there are well-documented side effects associated with prostaglandin analogs as a result of the research on Latisse as well as other glaucoma drugs.

Hair growth

Hair growth is a great side effect on your lashes! But if Latisse manages to make its way onto your cheek or brow bone, it has been reported to cause thicker and darker hair to grow in these areas. While this is not a harmful side effect, it could definitely be irritating.


Putting a new product on your eyes can definitely cause eye redness and irritation. With prostaglandin analogs, this can occur without irritation as prostaglandin acvitiy causes an inflammatory response. Among different prostaglandin analogs for glaucoma treatment, bimatoprost had the highest incidence of hyperemia (redness).

Periorbital hollowing (eye hollowing)

This is probably the least desirable side effect to come with the use of prostaglandin analogs. With the use of prostaglandin analogs, there have been multiple studies that show fat atrophy in the eye socket. What this means is that eyes begin to hollow and appear to “move backward” in the eye socket. This can increase the appearance of dark circles and be seen as a sign of aging.

Photo University of Iowa Dept. of Opthalmology

In this image, a patient used bimatoprost on only one eye (R) to treat glaucoma. The left eye is untreated.

This is just meant to provide a visual example- there are varying degrees of fat atrophy and “hollowing” that can come with prostaglandin analog use. In some patients, after use is discontinued, the eye socket regains its normal shape, but in others, the fat atrophy is permanent.

Iris or skin discoloration

The increased melanin production that is found in the hair follicle can also increase melanin production on the eyelids, or in rare cases, in the iris of the eye. This means that dark circles can become more prominent, and if you have light eyes, dark spots can occur. In those with brown eyes, the eyes can become darker.

Inoue et al., 2012.

A lot of the photos in case studies for this side effect are pretty bad- it’s hard to tell if photos are exactly the same when there are differences in lighting, angle, etc. This study (on glaucoma patients) showed that 50% of patients experienced iris discoloration, but the photos make this a little difficult to determine how severe the discoloration was.

It’s worth noting that the iris effects seem to be far more significant in populations applying prostaglandin analogs directly to the eye to treat glaucoma. For lashes, with proper application, the risk of iris discoloration seems to be quite low. However, this seems to be permanent, so if you love your eye color, definitely be cautious.

Personally, I would be open to trying a lash growth serum that doesn’t contain prostaglandin analogs if companies were transparent with the safety and efficacy data of their serum. (I hate wasting money, so I’d want to make sure it works first!)

My skin is sensitive to everything (even sunscreen!), so I would probably veer towards trying ones with fewer ingredients or ingredients I know aren’t a problem for me. If you’re not dealing with some kind of medical condition that causes hair loss, waterproof mascara (this one is BOMB) still may be the easiest way to make your lashes look longer and fuller for hot girl summer. Sorry to say it.

(If you’re struggling to take off your makeup, MakeUp Eraser cloths are a GAME CHANGER. I didn’t believe it until I tried it either.

how do lash growth serums work

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