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Rough rule: a noticeable increase sustained for more than two months, or visible scalp where there wasn't. Counting strands isn't useful — comparing photos of your parting a month apart is.

There's some evidence it helps circulation and reduces shedding, and it's genuinely useful for scalp health. It's not equivalent to medical treatment, and claims that it regrows a receded hairline are overstated.

Three months minimum, because that's the hair growth cycle. Anything promising results in two weeks is describing reduced breakage, not new growth.

Frequently, yes — iron deficiency, thyroid problems and vitamin D deficiency are all common causes of diffuse shedding, and they're all treated with a blood test rather than a serum. If shedding came on suddenly, get tested before you spend on products.

Patchy loss, a widening parting that keeps widening, shedding with pain or sores, or shedding that hasn't improved after three consistent months. Early treatment preserves hair that late treatment can't bring back.