Long eyelashes are primarily determined by your genetics, specifically by genes that control how long each lash grows before it falls out. Most people who notice unusually long lashes can thank their parents. But genetics isn’t the only factor: hormones, medications, nutrition, and occasionally underlying health conditions can all play a role in pushing lash length beyond the average.
What Controls Eyelash Length
Your eyelashes go through a growth cycle with three phases. The active growth phase typically lasts 30 to 45 days, which is dramatically shorter than scalp hair (which can grow for years). After that, the lash stops growing, detaches from its blood supply, and eventually falls out to make room for a new one. The length of your lashes depends almost entirely on how long that growth phase lasts and how fast the hair grows during it.
A key gene in this process is FGF5, which acts like a stop signal for hair growth. When FGF5 is working normally, it tells the follicle to stop growing and transition into the resting phase. Researchers studying families with exceptionally long eyelashes found that mutations in FGF5 disable that stop signal, allowing lashes to keep growing well beyond the usual window. In those families, the trait followed a recessive inheritance pattern, meaning a person needed to inherit the mutation from both parents to develop noticeably longer lashes.
That’s the extreme end of the spectrum. For most people, natural variation in how active FGF5 and related genes are simply shifts the dial. A slightly less aggressive stop signal means a slightly longer growth phase, which means slightly longer lashes. This is why long eyelashes tend to run in families even when nobody has unusually dramatic ones.
The Evolutionary Purpose of Lash Length
Eyelashes aren’t decorative. They function as an aerodynamic barrier that reduces airflow across the surface of the eye, cutting down on dust, debris, and evaporation. A study published in the Journal of the Royal Society Interface measured eyelash length across 22 species of mammals and found a remarkably consistent ratio: lash length is roughly one-third the width of the eye opening. That ratio, about 0.35, minimizes the air shear stress on the eye’s surface. Lashes shorter than that don’t provide enough protection. Lashes longer than that can actually funnel more air toward the eye rather than deflecting it.
Humans fall within this same range. So if your lashes seem long but still proportional to your eye size, they’re operating within the zone evolution selected for.
Hormones and Nutrition
Hormonal changes can influence lash thickness and growth rate, though the effect is subtler than with scalp or body hair. Prostaglandin receptors are present in the lash follicle during active growth, and natural fluctuations in prostaglandin levels (which happen during pregnancy, for example) can temporarily shift lash characteristics. Thyroid hormones also play a role in hair cycling throughout the body, including the eyelids. An overactive thyroid can sometimes produce thicker, faster-growing hair, while an underactive thyroid tends to cause thinning.
On the nutritional side, your lashes are made of keratin, the same protein as your fingernails and scalp hair. Producing keratin requires biotin (vitamin B7), iron, and adequate protein intake. A diet rich in these nutrients supports the follicle during its growth phase. Severe deficiencies in biotin or iron are more likely to cause lash loss than lash growth, but correcting a mild deficiency can sometimes make lashes appear fuller than before.
Medications That Increase Lash Length
If your lashes seemed to get longer after starting a new medication, that’s not your imagination. Several drug classes are known to cause eyelash growth as a side effect. The most common culprits are prostaglandin analog eye drops prescribed for glaucoma. These drops lower eye pressure, but they also extend the growth phase of the lash follicle, resulting in longer, thicker, darker lashes. Bimatoprost is the best-known example and is the only one also approved as a cosmetic lash-growth treatment.
Other medications linked to increased lash length include certain cancer drugs that target a growth factor receptor (EGFR inhibitors), cyclosporine (used for organ transplants and autoimmune conditions), and interferon alpha. If you’ve noticed a change in your lashes after starting any medication, it’s worth mentioning to your prescriber, not because it’s dangerous, but because it can signal how your body is responding to the drug.
When Long Lashes Signal a Health Condition
In rare cases, unusually long eyelashes, a condition called trichomegaly, can be associated with an underlying medical issue. Acquired trichomegaly has been reported in people with HIV infection, connective tissue disorders like lupus, and certain congenital syndromes. These conditions don’t cause long lashes in isolation. They’re almost always accompanied by other, more prominent symptoms.
For the vast majority of people wondering why their lashes are long, the answer is straightforward genetics. Trichomegaly linked to disease is rare enough to be the subject of individual case reports in medical journals. If you’re otherwise healthy and your lashes have always been long, or they’re similar to those of a close family member, a medical cause is very unlikely.
Sex, Ethnicity, and Perceived Lash Length
There’s a common perception that women have longer lashes than men, but measured differences between sexes are modest. What varies more is how lash length is perceived. Research on attractiveness preferences found that the most appealing lash length for female faces skewed slightly longer than for male faces, while very long lashes on men were rated as less attractive. This cultural framing may shape how people notice and evaluate their own lash length.
Across ethnicities, the preferred lash length consistently clusters around that one-third-of-eye-width ratio, though ratings for Black women’s faces skewed toward a preference for slightly longer lashes compared to other groups. The study found that men and women judge lash attractiveness in similar ways regardless of their own ethnicity, suggesting a broadly shared sense of what looks proportional.
Eye size also matters here. Someone with larger eyes will naturally have longer lashes to maintain that one-third ratio, which can make their lashes look striking even though the proportion is perfectly average. If your eyes are on the larger side, your “long” lashes may simply be doing exactly what biology intended.