Hair is dead protein produced by very alive follicles, and everything that matters happens at the follicle: its miniaturization under hormonal influence (pattern loss — the cause in most men and many women), its synchronized panic after body-wide stress (the dramatic shedding of telogen effluvium, which recovers on its own), and its autoimmune, traction, scarring and infective misadventures that need diagnosis rather than shampoo.
The evidence hierarchy here is unusually clean. Two treatments have large-trial proof of slowing loss and regrowing hair: topical minoxidil (both sexes) and oral finasteride (men) — decades old, unglamorous, effective for as long as they're used. Behind them: low-level laser devices with modest supportive trials, and a testable-cause list (iron, thyroid, crash diets, medications, postpartum shifts) whose correction fixes the shedding it caused. Behind that, at a great distance, the gummy-and-blend market — biotin's two tiny studies and lab-test problem, "DHT-blocking" botanicals leaning on saw palmetto's thin file, collagen's untested hair claims — sold hardest to exactly the people the proven pair would actually help.
The guides here map the diagnosis first — because pattern loss, shedding and patchy loss are different diseases with different fates — and grade the market second, with the standing rule that a widening part or a shedding pillow deserves a cause before it deserves a purchase.