Dry scalp or dandruff? They are not the same thing
This is the most useful distinction in the whole scalp category, and the shelf does almost nothing to help you make it.
Dry scalp is skin that is short on moisture, like dry skin anywhere else. The flakes are small, white and powdery, the scalp feels tight rather than greasy, it is often worse in winter or after over-washing, and the skin elsewhere on your body is probably dry too.
Dandruff is a different condition, associated with a yeast that lives on everyone's skin and with excess oil. The flakes are larger, yellowish and slightly greasy, and the scalp is oily rather than tight. The AAD's dandruff page is the right reference here (AAD, how to treat dandruff).
Buying an anti-dandruff shampoo for a genuinely dry scalp makes it worse — those formulas are designed to reduce oil on a scalp that has too much, and yours does not. That is the single most common wrong purchase in this category.
If it is dry scalp
The fix is mostly behavioral and it is free. Wash less often. Use lukewarm rather than hot water. Switch to a gentler, sulfate-free cleanser. Stop using dry shampoo daily, because it absorbs the oil your scalp needs as well as the oil it does not. Then add moisture if you still need to.
A scalp oil massaged in before washing can help, but it needs to come out properly afterwards — oil left on a dry scalp attracts everything else and becomes buildup.
If it is dandruff
You need an active ingredient, and the two here are salicylic acid, which lifts the scale physically, and selenium sulfide, which targets the underlying cause. Use it as directed, leave it on for the stated time rather than rinsing straight off, and expect to keep using it — dandruff is managed rather than cured.
If your scalp is greasy as well as flaking, you are in the other section: oily roots and scalp has the products built for that combination, and scalp scrubs handle the residue neither shampoo type reaches.
When to stop buying shampoo and see a doctor
If your scalp is sore, inflamed, bleeding, or shedding in patches, or if two months of consistent treatment with a proper active has changed nothing, that is a dermatology question rather than a shopping one. Psoriasis and seborrheic dermatitis both present as scalp flaking and both need treatment a cosmetic shampoo cannot provide.
We have not used these products ourselves. They are chosen from what is in the formula, what those ingredients are documented to do, and whether that matches the problem this page is about — and every product below links to the manufacturer's own page so you can check the formulation claims yourself. The full method is here.
How we picked
Products we have personally tested: none
Most hair care roundups are written in a "we tested these" voice. We have not, and saying so is the point. These picks are chosen the way an informed shopper would choose them: read what is actually in the formula, check what those ingredients are documented to do, match that against the specific problem the page is about, and confirm the product is still being sold.
Each pick gets a role — the drugstore option, the one for high porosity, the gentlest of the set — rather than a score out of ten, because a score would imply a measurement nobody took. Nobody pays for inclusion or placement, and when the cheap product is the right answer we say that too. The full method is on one page.