I have found two papers related to garlic for treating alopecia areata [1,2].
Is there any other evidence for hair growth in androgenetic alopecia?
In fact there are some statements [3,4] that it actually can cause hair loss.
[1] Hajheydari Z, Jamshidi M, Akbari J, Mohammadpur R. Combination of topical garlic gel and betamethasone valerate cream in the treatment of localized alopecia areata: A double-blind randomized controlled study. Indian J Dermatol Venereol Leprol 2007;73:29-32.
[2] Sharquie KE, Al-Obaidi HK. Onion juice (Allium cepa L.), a new topical treatment for alopecia areata. J Dermatol 2002 Jun; 29 (6): 343–6
[3] http://www.ehealthme.com/ds/garlic/hair+loss
[4] http://immortalhair.forumandco.com/t8184-aged-garlic-extract
Update:
What about the following link: Garlic increases nitric oxide [2] by Hydrogen Sulfide which it contains [3]. Minoxidil is also related to the generation of nitric oxide through vascular endothelial growth factor (VEGF) [1] and promotes hair growth. Can garlic have the same or similar or any effects on hair like Minoxidil via nitric oxide?
References: [1] Messenger, A. G., and J. Rundegren. "Minoxidil: mechanisms of action on hair growth." British journal of dermatology 150.2 (2004): 186-194.
[2] Ping-Ho Chen, Yaw-Syan Fu, Yun-Ming Wang, Kun-Han Yang, Danny Ling Wang, and Bin Huang, “Hydrogen Sulfide Increases Nitric Oxide Production and Subsequent S-Nitrosylation in Endothelial Cells,” The Scientific World Journal, vol. 2014, Article ID 480387, 8 pages, 2014. doi:10.1155/2014/480387
[3] http://www.sciencedaily.com/releases/2007/10/071016131534.htm