TLDR Tretinoin may help hair growth and works better when combined with minoxidil.
The study tested topical all-trans-retinoic acid (tretinoin) alone and in combination with 0.5% minoxidil on 56 subjects with androgenetic alopecia. After 1 year, 66% of subjects using the combination treatment experienced terminal hair regrowth, while 58% of those using only tretinoin saw some hair regrowth. One female subject with severe alopecia for over 20 years had significant hair regrowth after 18 months of tretinoin use. Tretinoin was found to promote cell proliferation and differentiation in the epithelium and may enhance vascular proliferation, which are important for hair growth. The results suggested further research on retinoids and their synergistic effects with minoxidil.
55 citations,
February 1985 in “Archives of Dermatology” Using minoxidil on the scalp can help grow hair for people with hereditary baldness.
80 citations,
September 1984 in “Journal of The American Academy of Dermatology” 5% minoxidil helps hair regrowth in androgenic alopecia.
137 citations,
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63 citations,
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February 2020 in “The Pharmaceutical Society of Japan” 
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56 citations,
April 2007 in “Journal der Deutschen Dermatologischen Gesellschaft” Minoxidil works better for female hair loss than alfatradiol, both safe.
January 2007 in “Strait Pharmaceutical Journal” 
1 citations,
September 2002 in “PubMed” Minoxidil helps stimulate hair growth and reduce hair loss in women with androgenic alopecia.
