Androgenetic alopecia (AGA) is the most common cause of hair loss and thinning in both men and women, characterized by follicular miniaturization as a result of the effects of systemic androgens (hormones involved in male traits and reproductive activity) and genetic factors. 
AGA remedies are fairly scarce and come with side effects. Interestingly, studies have found the endocannabinoid system (ECS) to play a key role in hair follicle cell growth control. More specifically, as a cannabinoid type 1 receptor (CB1) agonist, tetrahydrocannabinol (THC) “has been shown to dose-dependently inhibit hair shaft elongation, decrease proliferation of hair matrix keratinocytes and induce intraepithelial apoptosis and premature hair follicle regression (catagen).”  This makes THC helpful for unwanted hair growth but unhelpful for AGA.
On the other hand, as a CB1 antagonist, cannabidiol (CBD) makes for a hair growth stimulant in theory. This was tested by a group of scientists  who employed daily topical application of a CBD-rich hemp oil formulation.
The study involved 35 subjects—28 males and 7 females. The males’ average age was 43, whereas the females was 61. All had been diagnosed with AGA.
The participants were advised to apply a thin layer of the ~11% CBD hemp paste over the balding areas once each morning, which amounted to an average daily CBD dose of 3-4 mg. The trial lasted 6 months.
At the end of the 6-month period, the difference was noticeable.
In the temporal area, hair counts increased by 74.1% in men and 55.2% in women, on average. In the vertex (crown) area, the average increase was 120.1% for men and 64.9% for women. When looking at the temporal and vertex areas combined, the hair counts increased 93.5%.
“In general, males and the vertex area did the best. All subjects had some increase in hair counts.”
Moreover, side effects were negligible, virtually nonexistent in fact.
“One-third of the patients reported some slightly increased hair shedding during the first month of treatment, this was (sic) no longer was noted at the two-month visit.” The authors do note that high doses of CBD may negatively affect AGA due to activity through the vanilloid receptor-4 (TRPV4).
These promising results have prompted the scientists to perform another study, this time with a hemp extract that is high not just in CBD, but also tetrahydrocannabivarin (THCV) and cannabidivarin (CBDV), which are known to be potent CB1 antagonists. Considering the speculated power of the entourage effect, the results could be even more impressive. 
Image Credit: pxfuel
- Smith GL, Santino J. Hair regrowth with cannabidiol (CBD)-rich hemp extract – A case series. Cannabis. 2021;4(1). [Impact Factor N/A; Times Cited: 0]