I have very sensitive skin and usually react to everything out of the ordinary like chemical exfoliators. I already reacted to Klairs' Mandelic Acid. The only thing this one did was bring out pimples I already had, which is called "purging". It's not as bad as it sounds: it just made my pimples came out faster. One other thing I noticed: my blackheads are fading and I only used this twice. Great BHA for sensitive skin as expected from COSrx.