Cosmetics · Quasi-drug actives
4-Methoxysalicylic Acid Potassium Salt
4-メトキシサリチル酸カリウム塩 (4-Metokishi sarichirusan kariumu en)
Also known as: Potassium Methoxysalicylate, Shiseido refers to this ingredient as 4MSK
At a glance
| Category | Cosmetics |
|---|---|
| INCI name | Potassium Methoxysalicylate |
| Japanese labeling name | 4-メトキシサリチル酸カリウム塩 |
| Common Japanese notations | 4-メトキシサリチル酸カリウム塩, 4MSK |
| CAS number | 152312-71-5 |
| Origin | Synthetic (salicylic acid derivative) |
| Typical functions | Quasi-drug whitening (medicated bihaku) active |
| Regulatory status in Japan | Approved as a quasi-drug (医薬部外品) whitening active by Japan's MHLW. Approval applied for by Shiseido and granted in 2003. |
4-Methoxysalicylic acid potassium salt is a salicylic acid derivative approved as a quasi-drug whitening active in Japan. Shiseido — the company that applied for the approval, granted in 2003 — refers to the ingredient using the abbreviation 4MSK across its Japanese-market communications. The ingredient is positioned around prevention of melanin-related hyperpigmentation, the standard regulatory claim wording for medicated whitening actives.
Classification
Tags below link to other ingredients sharing the same attribute, so you can pivot from one ingredient to its peers.
Product applications
Functions
Regulatory tags
Origin
Used in (typical product categories)
Finished-product categories that commonly include this ingredient in Japanese-market formulations.
- Medicated whitening serums (薬用美白美容液)
- Medicated whitening lotions and emulsions
- Quasi-drug skincare lines
- Spot-care treatment products
What it is
4-Methoxysalicylic acid potassium salt is the potassium salt of 4-methoxysalicylic acid, a synthetically produced salicylic acid derivative. Within the Japanese quasi-drug regulatory framework it is permitted as an approved active ingredient at concentrations defined by Shiseido's MHLW approval.
Quasi-drug status means the ingredient appears in the active ingredient (有効成分) section of finished-product ingredient lists, separate from the other ingredients (その他の成分).
Typical uses in Japanese products
Medicated whitening (薬用美白) lines, particularly those marketed by Shiseido. The ingredient appears in serums, lotions, and emulsions positioned around prevention of sun-induced spots.
Regulatory classification in Japan
Approved by MHLW as a quasi-drug whitening active in 2003 following an application by Shiseido. Permits the standard medicated-whitening claim wording: prevention of spots (シミ) and freckles (ソバカス) caused by sun exposure.
Regulatory classification in other markets
| EU | Listed in CosIng under Potassium Methoxysalicylate. Permitted for cosmetic use; not the subject of a quasi-drug-equivalent regulatory category in the EU. |
|---|---|
| USA | INCI Potassium Methoxysalicylate. Used in cosmetic products. |
| China | Verify the specific preparation against current IECIC listings. |
| Korea | Permitted for cosmetic use under KFDA / MFDS. |
Example products
Example finished products containing this ingredient are marketed primarily by Shiseido under various medicated whitening lines. Specific SKU listings will be added after each product's current full ingredient list has been verified.
All brand names and product names referenced anywhere on this site are the property of their respective owners. Example entries are provided for informational purposes only and do not imply endorsement.
Related ingredients
References
- Japan PMDA — quasi-drug active ingredient approval registry
- Shiseido official corporate publications referencing 4MSK
Last updated: 2026-04-22. Ingredient entries are reviewed at least annually against current regulatory listings.