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Ensulizole

Ensulizole suppliers and sourcing guide

Ensulizole is a water-soluble UVB filter, 2-phenylbenzimidazole-5-sulfonic acid. It is made by synthesizing 2-phenylbenzimidazole and sulfonating it, then purifying the acid. It is supplied as a white to off-white powder that dissolves only after it is neutralized with a base such as sodium hydroxide, triethanolamine or tromethamine, so it goes into the water phase. In the U.S. it is an OTC sunscreen drug active.

See suppliersWhat to ask for on the COA
Also known as
phenylbenzimidazole sulfonic acid (INCI) · 2-phenylbenzimidazole-5-sulfonic acid · PBSA · CAS 27503-81-7
Facts checked
November 2026

Forms and grades of ensulizole

Suppliers sell different products under the same name. Specify the one you need.

  • USP / drug-grade acid powder

    Ensulizole free acid made to the USP monograph for OTC sunscreen use.

  • Cosmetic-grade acid powder

    Phenylbenzimidazole sulfonic acid sold as a cosmetic UV filter in other markets.

  • Pre-neutralized solutions

    Ensulizole supplied as an aqueous salt solution, which avoids an in-house neutralization step but must be dosed by its stated active content.

What to check on the COA

Compare each result with your own specification when the lot arrives.

Assay (HPLC or titration)
Ensulizole content on the dried basis.
Specific absorbance
UV absorbance as an identity and strength check.
Loss on drying
Water content, needed to dose the acid correctly.
Clarity and color of solution
Appearance of a neutralized test solution.
Related substances and heavy metals
Synthesis impurities by HPLC and metals by ICP-MS.

Storage, lots and expiry

Storage and shelf life

Keep ensulizole powder in sealed containers at room temperature, away from moisture. Neutralized solutions should be kept closed and used within the supplier shelf life. Retest dates come from the supplier specification.

Lots and expiry dates

Record assay and loss on drying for each lot, since both change how much base and powder a batch needs. Keep certificates with drug GMP records.

Regulatory status

Facts from regulators and standards bodies, each linked to its source.

  • 21 CFR 352.10 lists phenylbenzimidazole sulfonic acid (ensulizole) up to 4 percent among OTC sunscreen active ingredients; FDA describes this 1999 final sunscreen monograph as stayed (not in effect).

    Source: ecfr.gov (opens in a new tab)

  • FDA says the CARES Act deemed final order for OTC sunscreens (OTC000006) considers sunscreens containing the 16 active ingredients of that 1999 monograph as generally recognized as safe and effective (GRASE).

    Source: fda.gov (opens in a new tab)

  • FDA's 2021 proposed order (OTC000008) proposed that sunscreens containing ensulizole are not GRASE because additional data are needed; FDA says this is not a conclusion that the ingredient is unsafe and that the remaining proposals will be addressed in future orders.

    Source: fda.gov (opens in a new tab)

  • Health Canada's Primary Sunscreen Monograph lists ensulizole (2-phenylbenzimidazole-5-sulfonic acid) as a UVB filter at up to 4%.

    Source: webprod.hc-sc.gc.ca (opens in a new tab)

This is general information, not regulatory advice. Status differs by country and by how the ingredient is used, so check the rules for your product and market.

Ensulizole suppliers

Manufacturers and distributors whose own websites say they sell it.

We are checking ensulizole suppliers now

Every listing is checked against the supplier’s own website before it goes live, so this list fills in over the coming weeks. The specs and COA checklist above are ready today.

Sell ensulizole? Request a listing

Frequently asked questions

What forms of ensulizole can brands buy?

Common forms include uSP / drug-grade acid powder, cosmetic-grade acid powder and pre-neutralized solutions. Ask each supplier for the specification sheet of the exact grade they quote.

What should the COA for ensulizole include?

Expect assay (HPLC or titration), specific absorbance, loss on drying, clarity and color of solution and related substances and heavy metals, each with its method, specification and result for that lot.

How should ensulizole be stored?

Keep ensulizole powder in sealed containers at room temperature, away from moisture. Neutralized solutions should be kept closed and used within the supplier shelf life. Retest dates come from the supplier specification.

What is the regulatory status of ensulizole?

21 CFR 352.10 lists phenylbenzimidazole sulfonic acid (ensulizole) up to 4 percent among OTC sunscreen active ingredients; FDA describes this 1999 final sunscreen monograph as stayed (not in effect). FDA says the CARES Act deemed final order for OTC sunscreens (OTC000006) considers sunscreens containing the 16 active ingredients of that 1999 monograph as generally recognized as safe and effective (GRASE). FDA's 2021 proposed order (OTC000008) proposed that sunscreens containing ensulizole are not GRASE because additional data are needed; FDA says this is not a conclusion that the ingredient is unsafe and that the remaining proposals will be addressed in future orders. Health Canada's Primary Sunscreen Monograph lists ensulizole (2-phenylbenzimidazole-5-sulfonic acid) as a UVB filter at up to 4%.

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