Salicylic Acid

Salicylic acid is a beta hydroxy acid, or BHA, and it is the only exfoliating acid in common cosmetic use that is oil-soluble. That single property separates it from the alpha hydroxy acids such as glycolic and lactic acid, which are water-soluble and work across the surface. Being oil-soluble, salicylic acid can move through the sebum that fills a pore and act inside it.

It is related to salicin, a compound found in willow bark, and you will sometimes see willow bark extract marketed as a natural alternative. In practice the two are not equivalent: willow bark extract contains only small and variable amounts of the relevant compounds, and behaves quite differently from a formulated salicylic acid product.

On an ingredients list it appears as Salicylic Acid. In leave-on products in the UK it is typically used at 0.5% to 2%, with 2% being the familiar upper limit for over-the-counter face products. Rinse-off cleansers use similar figures but achieve much less, simply because contact time is short. Higher-strength professional peels exist but belong in a clinic setting.

You will meet it most often in products aimed at congested, oily or breakout-prone skin: cleansers, toners, leave-on liquids, targeted gels and pads. It also appears in body products for rough or bumpy-looking skin on the arms and back, and in some scalp and dandruff formulations.

It is worth noting that salicylic acid also has a mild calming effect of its own, which is part of why it tends to leave congested skin looking less angry as well as clearer, rather than simply stripped.

Salicylic acid does two things at once. Like other exfoliating acids, it loosens the bonds holding dead cells to the surface, so the skin sheds more evenly and looks smoother and less dull. Unlike the water-soluble AHAs, it does not stop at the surface: because it dissolves in oil, it travels down into the pore lining, where a mixture of sebum and shed cells accumulates and creates the congestion that shows up as blackheads, bumps and blemishes.

Inside the pore it helps break up that plug and encourages the pore to clear from within. This is why it is described as keratolytic, meaning it works on the build-up of keratin that blocks the opening, rather than simply polishing the top of the skin. Over consistent use, pores that stay clearer tend to look smaller, because much of what makes a pore look enlarged is what is sitting inside it.

It also has a modest calming action, related to its chemical kinship with aspirin, which helps explain why congested skin often looks less red and irritated during use rather than more so. And because it is mildly antibacterial in the pore environment, it makes that environment less hospitable to the bacteria associated with the look of breakouts. Effects on texture and oiliness usually show within a few weeks; the appearance of persistent congestion takes longer.

Salicylic acid is straightforward, but the most common mistake is using it everywhere, every day, on skin that does not need it.

• Strength: 2% is the standard for leave-on products and is not a beginner step. Starting at 0.5–1%, or using a 2% cleanser rather than a leave-on, is a gentler way in.
• Frequency: begin two or three times a week and build. Daily use suits oily, resilient skin; it is often too much for anything else.
• When: evening is the usual choice, though morning is fine if you are diligent with sunscreen.
• Order in the routine: after cleansing, onto dry skin, before serums and moisturiser. Give it a minute to settle.
• Spot-treat rather than blanket-treat. Many people only need it on the nose, chin and forehead. Applying it to dry cheeks that never break out simply causes irritation.
• Pairs well with: niacinamide, hyaluronic acid, panthenol, ceramides and azelaic acid.
• Keep apart from: retinoids, benzoyl peroxide, AHAs and scrubs on the same occasion. Alternate evenings instead, and never stack acids to work faster.
• Sunscreen: exfoliated skin is more exposed to daylight. SPF 30 or above every morning.
• If skin gets tight, shiny or sore, that is over-exfoliation, not progress. Stop, rebuild with a simple moisturiser for a week or two, then restart less often.

Give it six to eight weeks of steady use before judging what it is doing for congestion.