The brief transitional phase of hair follicle cycling — the growth phase switches off, the follicle shrinks, and the hair prepares to shed.Lasts approximately 2–3 weeks. Matrix cell proliferation stops; the follicle involutes. Neurogenic inflammation via substance P is proposed to prematurely push follicles into catagen in stress-related hair shedding (telogen effluvium). Understanding catagen is relevant to interpreting any compound claimed to prevent hair loss — if the mechanism doesn’t address premature catagen entry, it may not address the most common cause of shedding.