A thin, fast-absorbing acid serum built on lactic acid, with botanical sources of glycolic acid from sugarcane and salicylic acid from willow bark. Arbutin and mushroom extract sit alongside them for the look of a more even tone. There is menthol in the formula, which is why it cools and tingles on application. iS Clinical does not publish the acid percentages.

Suits most skin types, and it is our first choice for blemish-prone and congested skin. Three to four drops on clean, dry skin at night, before anything heavier. Do not layer it with a retinoid on the same evening, and give it a full four weeks. If the tingle turns to burning, you have used too much.