The following study is fairly widely cited as the basis for considering onion juice a viable alternative treatment for alopecia.

Onion juice (Allium cepa L.), a new topical treatment for alopecia areata.

J Dermatol. 2002 Jun;29(6):343-6.Sharquie KE, Al-Obaidi HK.
Department of Dermatology and Venereology, Baghdad Teaching Hospital, Iraq.

Alopecia areata is a patchy, non-scarring hair loss condition. Any hair-bearing surface may be involved, and different modalities of treatment have been used to induce hair regrowth. This study was designed to test the effectiveness of topical crude onion juice in the treatment of patchy alopecia areata in comparison with tap water.

The patients were divided into two groups. The first group [onion juice treated] consisted of 23 patients, 16 males (69.5%) and 7 females (30.5%). Their ages ranged between 5-42 years with a mean of 22 years. The second group [control; tap-water-treated] consisted of 15 patients, 8 males (53%) and 7 females (46%). Their ages ranged between 3-35 years with a mean of 18.3 years. The two groups were advised to apply the treatment twice daily for two months.

Re-growth of terminal coarse hairs started after two weeks of treatment with crude onion juice. At four weeks, hair re-growth was seen in 17 patients (73%), and, at six weeks, the hair re-growth was observed in 20 patients (86.9%) and was significantly higher among males (93%) compared to females (71%). In the tap-water treated-control group, hair re-growth was apparent in only 2 patients (13%) at 8 weeks of treatment with no sex difference.

The present study showed that the use of crude onion juice gave significantly higher results with regard to hair re-growth than did tap water, and that it can be an effective topical therapy for patchy alopecia areata.