Salicylic acid (BHA)

Measured on: acne lesion counts, comedonal and inflammatory.

Nothing published dates this ingredient. The salicylic-acid-specific citation in this base is not a trial but a manufacturer review: staff of the company selling the salicylic acid pads summarising four studies, three of them placebo-controlled but unpublished, with no timepoints given. The 2024 AAD acne guideline recommends topical salicylic acid only conditionally, on low-certainty evidence from a single randomised trial, and says nothing about when a user should expect lesion counts to fall. The modern randomised literature does not fill the gap: it is either a different exposure entirely (20-30% in-office peels), an active comparator with no vehicle arm (adapalene, benzoyl peroxide), or a multi-ingredient formula in which salicylic acid cannot be separated. The 8-12 week figure that circulates is the standard length of an acne trial and the interval at which clinicians reassess any acne therapy - a design convention carried over from better-studied molecules, not a measurement of this one. The keratolytic effect on an individual comedone is faster than any of that, and nobody has published it as a timed endpoint either. What is missing specifically: a vehicle-controlled trial of a 0.5-2% leave-on or cleanser reporting lesion counts visit by visit.

PMID:1535287 (Zander & Weisman, Clin Ther 1992;14(2):247-53 - a manufacturer-authored review of four studies, no timepoints) and PMID:38300170 (AAD 2024 acne guideline - conditional recommendation, low-certainty evidence from a single randomised trial, no onset statement); both read against the claim. No vehicle-controlled trial of 0.5-2% leave-on salicylic acid reporting lesion counts by visit was located on PubMed.