Whether the compounds found in cannabis, particularly cannabinoids, can influence the rate or density of hair growth is a common question. This topic involves biological interaction, where compounds like tetrahydrocannabinol (THC) and cannabidiol (CBD) engage with the body’s native systems. Understanding the scientific basis requires examining the normal cycle of hair growth and the specific mechanisms by which these cannabis components interact with the body’s cellular machinery. Current research provides insights into how these substances might affect hair follicles.
Understanding the Hair Growth Cycle
Hair growth is a carefully timed cycle that repeats throughout a person’s life, involving three main phases. The anagen phase is the active growth period where the cells in the hair follicle divide rapidly to form the hair shaft. This phase is the longest, typically lasting anywhere from two to seven years, and its duration determines the maximum length a hair can achieve.
The second phase is catagen, a short transitional stage lasting only about two to three weeks. During catagen, the hair follicle shrinks, and hair growth stops as the hair detaches from its blood supply. The cycle then enters the final main phase, telogen, which is a resting period for the follicle, lasting around three to four months.
After the resting phase, the old hair is shed in the exogen phase, and the follicle re-enters the anagen phase to begin growing a new hair. For any substance to promote longer or thicker hair, it must primarily act to extend the duration of the anagen phase or shorten the resting or transitional phases. Disruptions to this cycle can lead to noticeable hair loss or thinning.
How Cannabinoids Interact with the Body
The mechanism through which cannabinoids affect the body is centered on the Endocannabinoid System (ECS), a network of receptors and signaling molecules present throughout the human body. The primary components of this system are the cannabinoid receptors, specifically Cannabinoid Receptor Type 1 (CB1) and Cannabinoid Receptor Type 2 (CB2). CB1 receptors are found abundantly in the central nervous system, while CB2 receptors are more associated with the immune system.
Crucially, both CB1 and CB2 receptors are also present in the skin, including within the epithelial cells of hair follicles and in sebaceous glands. This presence means that phytocannabinoids like THC and CBD can directly interact with the structures responsible for hair production. The ECS in the skin helps regulate various cellular processes, including cell proliferation, differentiation, and survival, which govern the hair cycle.
Cannabinoids act on these receptors to modulate different functions, providing a direct pathway for them to influence the hair growth cycle. For example, THC acts as a partial agonist on the CB1 receptor, while CBD can function as a negative modulator of the CB1 receptor. Their distinct actions mean that the two compounds may have different, or even opposite, effects on the hair follicle.
Current Research on Hair Follicle Response
Research using human hair follicle cultures has provided specific details on how different cannabinoids affect the hair growth cycle, often revealing a complex, dose-dependent relationship. Studies show that tetrahydrocannabinol (THC), which activates the CB1 receptor, can inhibit hair shaft elongation and decrease the proliferation of hair matrix keratinocytes. This action suggests that THC may cause the hair follicle to enter the catagen, or regression, phase prematurely.
The effects of cannabidiol (CBD) are more nuanced and depend heavily on concentration. In laboratory settings, high concentrations of CBD, such as 1 to 10 micromolar, have been shown to have a negative impact on the hair cycle. These higher levels accelerate the premature entry of hair follicles into the catagen phase, likely through a different receptor known as TRPV4.
Conversely, some ex vivo studies suggest that very low concentrations of CBD may promote hair shaft elongation, possibly by acting to block the CB1 receptor. This suggests that the impact of CBD is highly specific to the amount used, with a narrow window for potential benefit. In a clinical case series, a topical hemp extract containing a low daily dose of CBD was associated with an average increase in hair count for patients with androgenetic alopecia, highlighting the importance of delivery method.
The distinction between systemic consumption (ingestion or smoking) and topical application is also significant because cannabinoids are fat-soluble and can reach the hair follicles more directly when applied to the scalp. Overall, the current scientific evidence does not support the general claim that cannabis reliably “makes hair grow.” Instead, it suggests that the primary psychoactive compound, THC, may inhibit growth, while the non-psychoactive compound, CBD, may only offer a potential benefit in highly specific and low topical concentrations.