Cetirizine is a second-generation H1 antihistamine, widely used for allergic rhinitis and urticaria. Its inclusion in the hair loss conversation came from observation that the drug has anti-inflammatory effects beyond H1 receptor blockade, including some interference with prostaglandin D2 pathway signalling, relevant given the PGD2 elevation documented in androgenetic alopecia scalp.

A 2018 Italian pilot study tested 1% topical cetirizine lotion in 70 patients with androgenetic alopecia over 24 weeks. The treatment group showed a mean increase of 11.7 terminal hairs per cm² versus 2.4 in the vehicle group. The effect size was unexpectedly large for a repurposed antihistamine. A 2021 replication in 60 patients showed more modest but still positive results, suggesting the original study's effect may have been somewhat exaggerated by smaller sample size variation.

The mechanism is incompletely characterised. Cetirizine inhibits eosinophil recruitment and reduces several inflammatory mediators in skin. It also has some activity on the PGD2 pathway, though less specifically than dedicated antagonists like setipiprant. Currently, compounded 1% topical cetirizine is occasionally prescribed by hair specialists as an adjunct, particularly in patients with inflammatory scalp features. The evidence base is too thin to recommend as first-line therapy, but it represents an inexpensive, well-tolerated option in the specialist toolkit.