Salicylic acid has a long history. In 1838 the Italian chemist Raffaele Piria produced it from salicin, a compound found in willow bark, and named it after the Latin word for willow, Salix, according to a history in the journal Plant Physiology. Aspirin came decades later, out of the same family of chemistry.

On skin, salicylic acid is a keratolytic, which means it loosens and helps shed the outer layer of dead cells. That is why it shows up in acne cleansers, and it is also why it works on a flaky scalp. In the US, the FDA's rules for dandruff, seborrheic dermatitis and psoriasis products allow salicylic acid at 1.8 to 3 percent as an active ingredient.

So there are really two kinds of salicylic acid shampoo. Medicated ones, like Neutrogena T/Sal at 3 percent, are dandruff and scaling treatments. Clarifying ones use salicylic acid alongside other ingredients to lift oil and product buildup. Both are useful, but they do different jobs.

The first grid is eight salicylic acid shampoos, from medicated to clarifying. The second is eight scalp scrubs, serums and pre-wash treatments that use salicylic acid or similar exfoliants.

My tip: treat it like an exfoliating face wash. Massage it into your scalp, leave it for a minute or two, then rinse, and use it a few times a week rather than every day.

Neutrogena T/Sal Therapeutic Shampoo is the medicated classic, with 3% salicylic acid, the top of the range the FDA allows. It is made for scalp build-up and scaling, and it is the one I would start with for stubborn flakes.

Oribe Serene Scalp Anti-Dandruff Shampoo uses 2% salicylic acid to fight flaking and soothe the scalp, and it is the most polished-feeling dandruff shampoo here. OUAI Anti-Dandruff Shampoo also uses 2% salicylic acid to relieve flaking, itching and irritation while leaving hair clean and soft.

The rest are gentler, everyday options. Moroccanoil Scalp Balancing Shampoo pairs salicylic acid with niacinamide to calm oil, flakes and itch. K18 PEPTIDE PREP Clarifying Detox Shampoo uses salicylic acid and charcoal to clear sebum buildup without stripping. Fenty Beauty's The Clear Thinker is a color-safe clarifying shampoo with salicylic acid for buildup and excess oil.

RŌZ Salt Scalp Scrub Exfoliating Detox Shampoo combines AHA and BHA exfoliation in a pH-balanced formula. CeraVe Oil Control Gentle Scalp Care Shampoo is a clinically tested formula that removes excess oil and cleans the scalp without drying it.

What I would skip: using T/Sal and a clarifying salicylic shampoo in the same week. Two exfoliating washes is too much for most scalps.

A lot of scalp exfoliation now happens before shampoo, and this grid is where that shows.

Act+Acre Salicylic Acid Scalp Renew Exfoliator is a 15-minute pre-wash treatment with 0.15% salicylic acid for oily, flaky scalps. Drybar Crown Tonic is a pre-shampoo cleanser with glycolic and salicylic acids. Dr. Groot Pre-Shampoo Exfoliating Detox Water targets buildup that regular shampoo does not remove. Dr.FORHAIR Aero Purifying Scalp Exfoliator is a mild liquid that dissolves built-up sebum.

For a physical scrub, Moroccanoil Purifying Pre-Wash Scalp Scrub uses argan shell powder with glycolic and salicylic acids, and Kérastase Energizing Scrub is made for oily scalps, with sea salt and vitamin B6. The Virtue Clarifying & Exfoliating Scalp Treatment is a light, dual-action formula that detoxifies the scalp.

AAVRANI Anti-Dandruff Scalp Serum is the leave-in, with salicylic acid to address irritation, itchiness, dryness, flaking and congestion. It is the one to use on the days you do not wash, a few drops along your part.

A pairing tip: use a pre-wash exfoliator once a week, followed by a gentle shampoo. You do not need a salicylic shampoo on the same day.

What I would skip: a scrub on a scalp that is already irritated or sunburned. Wait until it calms down, then start again.

I think salicylic acid shampoo is one of the most useful and least glamorous products in hair care. It does for your scalp what an exfoliating cleanser does for your face, and the chemistry behind it goes back almost two hundred years. Start with a medicated formula if you have real flakes, a clarifying one if you have oily buildup, and a weekly pre-wash if you want both. Give it a few weeks before you judge it, and see a dermatologist if flakes do not improve.