Hair care
Scalp hair grows, on average, at a rate of about 1.25 centimeters per month, and shampoos or vitamins have not been shown to noticeably change this rate. Hair growth rate also depends upon what phase in the cycle of hair growth one is actually in; there are three phases. The speed of hair growth varies based upon genetics, gender, age, hormones, and may be reduced by nutrient deficiency (i.e., anorexia, anemia, zinc deficiency) and hormonal fluctuations (i.e., menopause, polycystic ovaries, thyroid disease).
Prostaglandin D2
A 2012 research paper indicates a causal link between elevated levels of localized PGD and hair growth inhibition. Applied topically, the researchers found PGD prevents hair growth, and mice that were genetically inclined to produce higher levels of PGD had inhibited hair growth. The researchers also found PGD levels were much higher in balding scalp tissue than nonbalding scalp tissue, through increased levels of prostaglandin D2 synthase. The paper suggested that inhibition of hair growth involved binding of PGD to a receptor called GPR44, and that GPR44 therefore would be a therapeutic target for androgenic alopecia in both men and women with hair loss and thinning. Because PGD's relation to asthma has been known for several years, several drugs that seek to reduce the effect of PGD through blocking the GPR44 are already in clinical trials.