Eyelashes serve a specific function, acting as a sensory shield for the eye, much like the whiskers on a cat. These small hairs are designed to sense potential danger, triggering a protective blink reflex to keep dust, debris, and excessive airflow away from the delicate ocular surface. A sudden, noticeable change in their length or density can be surprising, signaling that a biological process or an external factor has altered the typical hair growth pattern. Understanding the reasons for this accelerated growth requires looking closely at the natural life cycle of the lash and factors that can interfere with it.
Understanding the Eyelash Growth Cycle
Like all hair on the body, an eyelash goes through a continuous, three-phase cycle of growth, transition, and rest. The cycle is significantly shorter than the cycle for hair on the scalp, which is why eyelashes do not grow indefinitely long. The first phase is Anagen, the period of active growth, which typically lasts only about 30 to 45 days for an eyelash.
The second stage is the Catagen phase, a brief transitional period lasting around two to three weeks where the hair follicle shrinks and growth ceases. Finally, the Telogen phase, or resting phase, can last for 4 to 8 weeks before the lash falls out naturally, making room for a new lash to begin the Anagen phase. Since only a small percentage of lashes are in the active growth phase at any time, a sudden change in length or thickness indicates that the duration of the Anagen phase has been artificially extended.
External Causes from Products and Serums
The most common reason for unexpected eyelash lengthening is the topical application of serums containing specific active ingredients. Many effective eyelash growth serums rely on prostaglandin analogs, which are synthetic compounds chemically similar to naturally occurring fatty acids in the body. These compounds were initially discovered to have this side effect when they were used in eye drops for treating glaucoma, a condition characterized by elevated pressure in the eye.
Prostaglandin analogs, such as bimatoprost or isopropyl cloprostenate, work by directly acting on the hair follicle. Their primary mechanism is to prolong the Anagen phase of the eyelash growth cycle. By extending this active growth period beyond its natural limit, the lashes have more time to grow longer and thicker than they otherwise would. This mechanism can also stimulate melanin production within the follicle, which often results in the lashes appearing darker as well as longer.
It is also possible to experience this effect unintentionally. Accidental exposure can happen if a person is using prostaglandin-based eye drops for glaucoma treatment, where the medication comes into contact with the lash line. Moreover, some over-the-counter eyelash serums may contain these same powerful prostaglandin analogs under different chemical names or derivatives. These cosmetic products can produce the same noticeable growth, leading to the impression that the eyelashes are suddenly growing without any clear cause.
Internal Conditions and Systemic Medications
Beyond topical products, a change in eyelash growth can be a localized manifestation of a systemic change in the body’s chemistry. Hormonal fluctuations are a significant internal factor that can influence hair follicle activity, including those of the eyelashes. For instance, periods of elevated estrogen, such as during pregnancy, are known to extend the Anagen phase of hair growth, potentially resulting in longer, thicker lashes.
In some cases, thyroid disorders can also impact hair growth patterns, although they are more often associated with hair loss and thinning. Any significant imbalance in thyroid hormones can disrupt the normal synchronization of the hair cycle across the body, which may include the eyelashes. The condition of excessive eyelash growth, medically termed trichomegaly or hypertrichosis, can also be a side effect of certain systemic medications taken for non-cosmetic purposes.
The most well-known of these systemic causes are the same prostaglandin analogs that are used in eye drops, as their effect on hair growth is not strictly localized. Other classes of medication, including specific drugs used in chemotherapy, such as Epidermal Growth Factor Receptor (EGFR) inhibitors, are known to have excessive hair growth as a recognized side effect. Even some immunosuppressants, like cyclosporine or tacrolimus, and certain anticonvulsants, such as phenytoin, have been linked to causing hypertrichosis in patients. Any unexpected, dramatic, or asymmetric growth warrants a discussion with a healthcare provider to determine if it is due to a medical condition or a systemic treatment.