The ingredient that actually makes eyelashes grow longer is bimatoprost, a prostaglandin analog. It’s the active compound in Latisse, the only FDA-approved prescription treatment for short or thin eyelashes, and it works by pushing resting hair follicles into their active growth phase and keeping them there longer than normal. Over-the-counter lash serums use a range of other ingredients, from prostaglandin-adjacent compounds to peptides and conditioning oils, but their effects vary widely and none have FDA approval for eyelash growth.
How Bimatoprost Triggers Lash Growth
Your eyelashes cycle through three phases: an active growth phase lasting roughly 30 to 45 days, a transition phase of two to three weeks where growth stops, and a resting phase before the lash eventually falls out. At any given time, many of your follicles are sitting in that resting stage, doing nothing.
Bimatoprost interacts with prostaglandin receptors in the hair follicle to do two things. First, it wakes up resting follicles and shifts them into active growth. Second, it extends the growth phase itself, giving each lash more time to get longer before it stops. The combined effect is lashes that are measurably longer, thicker, and darker. Latisse contains bimatoprost at a concentration of 0.03% and was first approved by the FDA in 2001 for ophthalmic use. Its lash-growing effect was actually discovered as a side effect in glaucoma patients using bimatoprost eye drops.
The Prostaglandin Analog in OTC Serums
Many popular over-the-counter lash serums contain isopropyl cloprostenate, a different prostaglandin analog that is not FDA approved. It’s chemically related to bimatoprost and is said to work through a similar mechanism, prolonging the growth phase and stimulating keratin production. But because these products haven’t undergone FDA testing, their safety profile and actual effectiveness remain unverified, according to the American Academy of Ophthalmology.
If you see a lash serum advertising dramatic growth results without a prescription, check the ingredient list. Isopropyl cloprostenate (sometimes listed under similar names like dechloro dihydroxy difluoro ethylcloprostenolamide) is the ingredient doing the heavy lifting. Without it or another prostaglandin-related compound, the product likely isn’t capable of making lashes physically grow longer.
Peptides and Growth Factors
Some serums skip prostaglandins entirely and rely on peptides, which are short chains of amino acids designed to signal specific cellular activity. Two that appear frequently are myristoyl pentapeptide-17 and myristoyl hexapeptide-16. These peptides stimulate keratin production, the structural protein that hair is made of. Myristoyl pentapeptide-17 also helps deliver other active ingredients in a serum deeper into the lash follicle.
Peptide-based serums generally produce more modest results than prostaglandin analogs. They work primarily by strengthening the lash itself, reducing breakage, and improving the bonds within the follicle rather than fundamentally altering the growth cycle. The result is lashes that look fuller because fewer are snapping off prematurely, not necessarily because they’re growing from new follicles. For people who want to avoid the side effects associated with prostaglandins, peptide serums offer a gentler alternative with a lower risk profile.
Conditioning Ingredients That Don’t Grow Lashes
A large category of lash serums relies on hydrating and strengthening ingredients like castor oil, coconut oil, argan oil, biotin, panthenol, hyaluronic acid, and soybean oil. These ingredients nourish the lash and prevent breakage, which can make lashes appear fuller and lusher over time. But they do not stimulate actual growth. An ophthalmologist-developed serum made of castor oil, coconut oil, and argan oil, for example, promotes the appearance of thicker lashes by keeping them hydrated and reducing brittleness, not by activating dormant follicles.
This distinction matters. If your lashes are naturally fine but healthy, conditioning ingredients alone won’t change their length. If your lashes look thin because they’re dry, brittle, and breaking mid-cycle, a good conditioning serum can make a real visible difference without any growth-stimulating compounds at all.
Botanical Compounds Like Redensyl
Redensyl is a blend of botanical ingredients that shows up in some newer lash and hair products. Its active components are two stabilized polyphenols that target stem cells in the outer root sheath and the dermal papilla of the hair follicle, essentially nudging follicle stem cells into greater activity. The blend also includes glycine and zinc, which support hair metabolism. One study using a combination of Redensyl, capixyl, and another botanical compound showed 64.7% improvement in hair growth after 24 weeks, though this was studied on scalp hair rather than eyelashes specifically. The translation to lash growth is plausible but less established than the evidence behind prostaglandin analogs.
Side Effects of Prostaglandin-Based Serums
Prostaglandin analogs are the most effective lash growth ingredients available, but they carry real side effects. More than 10% of patients treated with bimatoprost develop a condition called prostaglandin-associated periorbitopathy. Symptoms can include deepening of the upper eyelid crease, loss of orbital fat around the eye socket, drooping eyelids, and darkening of the skin around the eyes. These changes can make the eye area look sunken or hollow.
There’s also a risk of permanent iris color change. Bimatoprost can gradually darken the colored part of the eye, particularly in people with mixed-color irises like hazel or green-brown. Both the iris color change and changes to the eyelid skin can be permanent, even after you stop using the product. This risk applies to prescription Latisse and potentially to OTC serums containing other prostaglandin analogs, though the exact risk with isopropyl cloprostenate is less studied.
How Long Results Take
Regardless of which type of serum you use, visible results take time. Most people notice early changes around weeks four to eight, with lashes appearing more uniform and less brittle. More defined, noticeably fuller results typically appear after 10 to 12 weeks of consistent daily application. The first couple of weeks are essentially a conditioning phase where changes are happening at the follicle level but aren’t visible yet.
Consistency matters more than most people expect. Lashes cycle naturally, and a serum needs to be present through multiple growth cycles to maintain results. If you stop using a prostaglandin-based serum, your lashes will gradually return to their original length and thickness as the growth cycle normalizes. Conditioning serums require ongoing use for the same reason: once you stop protecting lashes from breakage, the visible fullness fades.