Potassium Methoxysalicylate
4MSK
No meaningful concern for most people; irritation is possible but rare.
Also written as: Potassium 4-Methoxysalicylate · 4MSK
What it is
Potassium methoxysalicylate is the potassium salt of 4-methoxysalicylic acid, usually shortened to 4MSK. It was developed in Japan as a quasi-drug brightening active, where it is used to reduce uneven pigmentation and support normal turnover of the top skin layer. Because the acidic group is neutralised and the ring is methoxylated, it is far less acidic and less irritating than salicylic acid itself.
What it does well
Milder on skin than salicylic acid
It is supplied as a neutral salt rather than a free acid, so formulas do not need the low pH that drives salicylic acid's sting. Most published work is Japanese industry research on pigmentation rather than independent trials.
Limited evidence
Worth knowing
Little independent evidence outside Japan
Its regulatory history is as a Japanese quasi-drug active; there is no equivalent EU assessment, and published data is limited and largely manufacturer-generated.
Matters most toAnyone paying a premium for it as a headline brightening active.
Where the EU stands
No specific restriction
Not listed in any Annex of the EU Cosmetics Regulation. The 2% Annex III limit on salicylic acid applies to salicylic acid itself, not to this methoxylated derivative.
