Onion (Allium cepa L.)-derived preparations ,including onion juice and onion oil have been increasingly investigated as phytotherapeutic agents for scalp and hair wellness due to their rich phytochemical profile (sulfur-containing compounds, flavonoids, vitamins and trace minerals). Several clinical and preclinical studies report that topical application of onion juice or onion formulations can improve hair regrowth in patchy alopecia areata, reduce microbial load on the scalp, and support follicular health through antioxidant and anti-inflammatory effects. Mechanistically, Allium cepa constituents are proposed to enhance local microcirculation and nutrient delivery to hair bulbs, support keratin synthesis via sulfur donation, and exert antimicrobial/antifungal activity that helps control dandruff and scalp infections. Formulation factors (concentration, carrier oils, extraction method, and co-ingredients) strongly influence efficacy and tolerability; standardized extracts and controlled clinical trials remain limited. This review synthesizes clinical trials, animal studies, and phytochemical analyses to present an evidence-based appraisal of onion-based hair therapeutics, identifies knowledge gaps, and suggests priorities for standardized formulation development and rigorous clinical evaluation.