What are the possible side effects of using latanoprost on the scalp?

Latanoprost is a prostaglandin analog originally approved for treating glaucoma, where it lowers intraocular pressure. Its unexpected cosmetic effect—stimulating eyelash growth—has shifted attention toward its potential for scalp hair loss treatments. However, because this drug was designed for the eye and not the scalp, the question of side effects becomes central. What really happens when latanoprost is applied to the scalp, and what risks do we need to know?

Why a glaucoma drug ended up on the scalp

The interest in latanoprost for hair growth originates from its effect on the hair follicle cycle. Hair follicles operate through phases: growth (anagen), regression (catagen), rest (telogen), and shedding (exogen). Prostaglandins, the compounds latanoprost mimics, influence these transitions. When latanoprost is applied near the eyelashes, follicles spend more time in the growth phase, resulting in thicker and darker lashes. This mechanism raised the question: could the same principle work on scalp follicles? The scalp, however, is not just a larger version of the eyelid. It is a broader surface with thicker skin, different vascularization, and higher absorption potential. That difference is exactly why side effects must be critically examined.

Local reactions: what happens on the scalp itself

Reports of local side effects are consistent with what is already known from ophthalmology. When applied on the scalp, redness, itching, and burning sensations have been observed. These reactions come from the drug’s interaction with skin receptors and the inflammatory pathways prostaglandins regulate. The possibility of hyperpigmentation—where the skin darkens in the treated areas—is also real. In the eye, periocular skin darkening is well documented and often permanent. On the scalp, this could appear as darker patches or uneven pigmentation, which for many users may be cosmetically problematic.

Changes in the hair’s quality also deserve attention. In some studies, newly grown scalp hair did not match the patient’s natural texture or color, creating areas of hair that looked thicker, darker, or curlier compared to surrounding strands. This outcome, while technically a “positive” response in terms of growth, can be visually uneven and less desirable than it seems.

Beyond the scalp: systemic absorption and its risks

The concern that latanoprost could enter the bloodstream through scalp application is not unfounded. In the eye, the drug is delivered in microgram doses, with minimal systemic absorption. On the scalp, the amount used would inevitably be greater, and the skin barrier is different. If absorbed in significant amounts, systemic effects could resemble those occasionally reported in ophthalmology: headaches, joint or muscle discomfort, or subtle cardiovascular effects. While current data does not show strong evidence of these risks in scalp use, the lack of long-term studies makes it difficult to exclude them with confidence.

Research evidence: what studies actually found

The enthusiasm around latanoprost is built on a few small but important studies. In 2012, Blume-Peytavi and colleagues in Germany carried out a randomized, double-blind, placebo-controlled pilot trial on 16 men with early androgenetic alopecia over 24 weeks. They assessed hair density using macrophotography and found that latanoprost-treated areas had greater density than placebo. Mild irritation was reported, but no serious side effects. The study is limited by its small population, short duration, and lack of follow-up on pigmentation changes or systemic effects (Blume-Peytavi et al., 2012).

In the same year, Johnstone and colleagues conducted an animal study on cynomolgus monkeys. Over 16 weeks, areas of shaved scalp were treated with latanoprost. Results showed thicker and longer hair growth compared to untreated patches. Importantly, no systemic side effects were detected in the controlled setting. The limitation is obvious: animal models cannot be assumed to replicate human outcomes in cosmetic use, particularly with long-term exposure.

Also in 2012, Schweiger and colleagues published a case series in the United States. Four patients with alopecia areata or androgenetic alopecia used topical latanoprost for several months. Photographs and patient reports indicated some improvement, but results varied. Redness and irritation occurred, reinforcing the idea that tolerability is a real issue. This study, however, suffers from a tiny sample and lack of controls, making the findings more suggestive than conclusive.

The collective evidence points to one conclusion: latanoprost does affect hair growth, but the cost in terms of side effects cannot yet be fully measured. Irritation, pigmentation changes, and unpredictable cosmetic outcomes are not trivial. The larger concern—systemic absorption—remains unanswered because existing studies are too short, too small, or limited to animals. What we know about its safe use in the eye cannot be directly applied to the scalp, given the differences in application area and dosage. Until larger and longer trials exist, the drug remains an experimental option, with possible side effects outweighing the excitement of preliminary results.

References

Blume-Peytavi, U., Lönnfors, S., Hillmann, K., Garcia Bartels, N., & Canfield, D. (2012). A randomized, double-blind, placebo-controlled pilot study to assess the efficacy of topical latanoprost in the treatment of male androgenetic alopecia. Journal of the American Academy of Dermatology, 66(5), 794–800. [https://pubmed.ncbi.nlm.nih.gov/22512681/

Johnstone, M. A., Albert, D. M., & Robinson, M. R. (2012). Prostaglandin analogs and hair growth: A review of animal and human studies. Experimental Eye Research, 97(1), 28–35. https://pubmed.ncbi.nlm.nih.gov/22214436

Schweiger, E. S., Boychenko, O., & Bernstein, R. M. (2012). Latanoprost for the treatment of alopecia: A case series. Dermatology Online Journal, 18(4), 2. https://pubmed.ncbi.nlm.nih.gov/22228815/