Cinnamon contains the active ingredient cinnamaldehyde. In one study cinnamon was found to have very similar hair growth stimulating results to 2% topical Minoxidil:
The application of topical cinnamon essential oil increases hair length and diameter size of male Wistar rat hair follicles as effectively as topical minoxidil 2%
The content of Cinnamaldehyde in cinnamon essential oil can prevent excessive hair loss in people with advanced age because Cinnamaldehyde increases vasodilation
of blood vessels causing an increase in blood flow, nutrition and oxygen to the hair roots, accelerating hair growth. Cinnamaldehyde can also stimulate IGF-1 metabolism in the papilla of hair follicles so that it can extend the anagen phase.
The study doesn’t detail how the chemical increases vasodilation. So I looked into whether it works by increasing VEGF, which is how several hair loss treatments increase blood circulation. According to one study, cinnamon decreases the growth factor VEGF. Since VEGF is actually required for hair growth (for keeping hair in the anagen (growth) phase), I’d be wary of using topical cinnamon for hair growth for this reason.
Consistent with these results, we observed significant suppression of VEGF expression, blood vessel formation, and tumor growth in a human ovarian tumor model in mice treated with CE