We know that the scalps of male pattern baldness patients have low PGE2 and high PGD2. We can reduce PGD2 using various chemicals and plant extracts. In conjunction with decreasing PGD2 it is even more beneficial for hair growth to increase PGE2. You can buy PGE2 and apply it to the scalp directly, but it’s extremely expensive. Therefore bitter orange, which is extremely cheap by comparison — and not toxic — seems to be a good choice for increasing scalp PGE2.
Here’s the relevant abstract from the researchers who studied limonene’s effect on PGE2:
In addition, treatment with D‑limonene significantly increased antioxidant, inducible nitric oxide synthase (iNOS) and cyclooxygenase‑2 (COX‑2) protein expression levels in UC rats. A decrease in prostaglandin E2 (PGE2) production, transforming growth factor‑β (TGF‑β) gene expression and an increase phosphorylated‑extracellular signal regulated kinase (ERK) 1/2 expression levels were observed in UC rats treated with D‑limonene.
Interesting that it was also shown to “significantly increase antioxidant, iNOS and COX-2 protein expression levels”.