The claim rests on one link in a chain. Willow bark contains salicin. In the body, swallowed salicin is metabolized to salicylic acid — that history is why willow bark is the folk ancestor of aspirin. Skincare marketing takes that fact and implies the same thing happens in a cream.
It does not, in any useful way. Converting salicin to salicylic acid requires metabolism that the skin surface does not perform efficiently. Even if it did, the arithmetic defeats it: a product might contain a fraction of a percent of willow bark extract, of which only part is salicin. A real BHA product contains 0.5 to 2 percent salicylic acid outright. The two are not in the same range, and the gap is large enough that it is not a matter of formulating harder.
What willow bark does bring is genuine, if modest. The tannins in it are mildly astringent, and its polyphenols have antioxidant and anti-inflammatory activity in the way most plant polyphenols do. On sensitive skin that is a reasonable thing to have in a formula. It is a supporting player.
Willow bark extract is well tolerated. It is mild, and most reactions to products containing it are caused by something else in the bottle.
People with a serious aspirin or NSAID allergy. The risk from a topical extract is theoretical and probably very small, but the ingredient offers little enough that it is not worth testing.
People who react to botanical extracts generally. If your skin flares on plant-heavy products, willow bark is in that family.
People who need an actual exfoliant. This is not a safety exclusion — it is the wrong product for the job, which is the more common way willow bark disappoints people.
Children, teenagers and anyone recovering from chickenpox or flu should not take willow bark by mouth. Salicylates by mouth in that setting carry a rare risk of Reye's syndrome. This applies to willow bark supplements and teas, not to a serum on your face.
Pregnancy: oral willow bark should be avoided, as oral salicylates generally are. A cosmetic containing a trace of the extract on your skin is not a concern.
Set expectations low and you will not be disappointed.
Immediately. A faint tingle in some formulas, and the general feel of the product. Nothing more.
Two to four weeks. If a willow bark product is going to help your skin look calmer or less red, this is where you would notice it, and the change will be subtle.
Six to eight weeks. This is a fair trial for any skincare product. If your blackheads, congestion or acne look the same, that is the expected result — the ingredient was never going to do that. Switch to salicylic acid, adapalene or benzoyl peroxide depending on the problem.
When you stop. Nothing changes and nothing rebounds. There is no adjustment period in either direction.