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Medical Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice.
Other
Phenylbenzimidazole Sulfonic Acid, also known as Ensulizole, is a water-soluble organic compound used as a UV-B filter in sunscreens and cosmetics to protect the skin from sunburn and long-term UV damage.
Name
Phenylbenzimidazole Sulfonic Acid
Raw Name
PHENYLBENZIMIDAZOLE SULFONIC ACID
Category
Other
Drug Count
6
Variant Count
6
Last Verified
February 17, 2026
About Phenylbenzimidazole Sulfonic Acid
Phenylbenzimidazole Sulfonic Acid, also known as Ensulizole, is a water-soluble organic compound used as a UV-B filter in sunscreens and cosmetics to protect the skin from sunburn and long-term UV damage.
Detailed information about Phenylbenzimidazole Sulfonic Acid
This page is for informational purposes only and does not replace medical advice. Consult a qualified healthcare professional before using any medication containing Phenylbenzimidazole Sulfonic Acid.
Phenylbenzimidazole Sulfonic Acid, commonly referred to in the cosmetic and dermatological industry as Ensulizole, is a potent, water-soluble organic compound that serves as a primary ultraviolet B (UV-B) filter. It belongs to the class of heterocyclic organic compounds known as benzimidazoles. Unlike many other organic sunscreen agents that are oil-soluble, Phenylbenzimidazole Sulfonic Acid is unique because it can be dissolved in the aqueous (water) phase of topical formulations. This specific characteristic makes it an indispensable ingredient for the development of 'oil-free' sunscreens, lightweight daily moisturizers, and clear gel-based sun protection products that do not leave a greasy residue on the skin.
Historically, the FDA (U.S. Food and Drug Administration) has recognized Phenylbenzimidazole Sulfonic Acid as a Generally Recognized as Safe and Effective (GRASE) Category I sunscreen ingredient. Under the FDA OTC Sunscreen Monograph (21 CFR 352), it is approved for use in over-the-counter sunscreen products at concentrations up to 4%. Globally, it is widely utilized, with the European Union's SCCS (Scientific Committee on Consumer Safety) and regulatory bodies in Australia (TGA) and Japan also permitting its use in similar concentrations. Its primary clinical role is the prevention of erythema (sunburn) caused by exposure to UV-B radiation, which ranges from 290 to 320 nanometers in wavelength.
The mechanism of action for Phenylbenzimidazole Sulfonic Acid is centered on its ability to act as a chemical UV filter. At the molecular level, the compound contains a conjugated system of double bonds within its benzimidazole ring structure. When UV-B photons strike the skin, the electrons within the Phenylbenzimidazole Sulfonic Acid molecule absorb this high-energy radiation. This absorption causes the electrons to transition from a 'ground state' to an 'excited state.'
Instead of allowing the UV energy to penetrate deeper into the skin—where it could cause DNA damage, oxidative stress, and the formation of pyrimidine dimers—the molecule dissipates this energy. It typically releases the absorbed energy as harmless lower-energy infrared radiation (heat) as the electrons return to their ground state. Because it specifically targets the UV-B spectrum (280–320 nm), it is exceptionally effective at preventing the immediate inflammatory response known as sunburn. However, because it provides minimal protection against UV-A (long-wave) radiation, it is almost always formulated alongside UV-A filters like Avobenzone or Zinc Oxide to provide 'Broad Spectrum' protection.
Phenylbenzimidazole Sulfonic Acid is exclusively used for topical photoprotection. Its FDA-approved indications include:
This ingredient is not available as a standalone drug but is a component in various topical formulations:
> Important: Only your healthcare provider or a board-certified dermatologist can determine if products containing Phenylbenzimidazole Sulfonic Acid are right for your specific skin type and medical history.
For effective photoprotection, Phenylbenzimidazole Sulfonic Acid must be applied at a specific density. The standard clinical recommendation, supported by the American Academy of Dermatology (AAD), is the application of 2 milligrams of product per square centimeter (2 mg/cm²) of skin.
No dosage adjustments are required for patients with kidney disease, as topical absorption is negligible.
No dosage adjustments are required for patients with liver disease.
Elderly patients often have thinner skin (dermal atrophy). While no specific dose reduction is needed, healthcare providers recommend gentle application to avoid skin tears, and more frequent reapplication may be necessary if the skin is excessively dry.
If you forget to apply sunscreen before sun exposure, apply it as soon as you remember. However, be aware that the skin may have already sustained UV damage. Seek shade immediately until the product has had time to set.
Topical 'overdose' of Phenylbenzimidazole Sulfonic Acid is not a recognized clinical condition. However, excessive application can lead to localized skin irritation or 'pilling' of the product. In case of accidental ingestion, especially by children, contact a Poison Control Center immediately. Symptoms of ingestion may include gastrointestinal distress or nausea due to the other inactive ingredients (excipients) in the sunscreen formulation.
> Important: Follow your healthcare provider's or the product label's dosing instructions. Do not adjust your application frequency without medical guidance.
Phenylbenzimidazole Sulfonic Acid is generally well-tolerated. The most common issues are localized to the site of application:
While extremely rare for a topical UV filter, systemic reactions can occur.
> Warning: Stop using Phenylbenzimidazole Sulfonic Acid and call your doctor immediately if you experience any of these:
There is currently no clinical evidence suggesting that long-term topical use of Phenylbenzimidazole Sulfonic Acid leads to systemic toxicity, endocrine disruption, or organ damage. The primary 'long-term' risk is a false sense of security; if a user relies solely on a UV-B filter without UV-A protection, they may avoid sunburn but still accumulate long-term DNA damage from UV-A rays. Regular dermatological check-ups are recommended for anyone with a history of heavy sun exposure.
No FDA black box warnings exist for Phenylbenzimidazole Sulfonic Acid. It is considered safe for over-the-counter use within the approved concentration limits.
Report any unusual symptoms or persistent skin changes to your healthcare provider.
Phenylbenzimidazole Sulfonic Acid is for external use only. It should never be ingested or applied to mucosal membranes (inside the mouth, nose, or genital area). Users should be aware that no sunscreen provides 100% protection against all UV radiation. It should be used as one component of a comprehensive sun protection strategy that includes wearing protective clothing, hats, and sunglasses, and seeking shade during peak UV hours (10 AM to 4 PM).
No FDA black box warnings for Phenylbenzimidazole Sulfonic Acid.
There are no standard laboratory monitoring requirements (such as blood tests) for the use of topical Phenylbenzimidazole Sulfonic Acid. However, patients should perform:
Topical application of this ingredient does not impair the ability to drive or operate heavy machinery.
There are no known interactions between topical Phenylbenzimidazole Sulfonic Acid and the consumption of alcohol. However, alcohol consumption can lead to dehydration and may increase the risk of heat-related illnesses during sun exposure.
If a rash or irritation develops, discontinue use immediately. There is no withdrawal syndrome associated with stopping sunscreen use, but the risk of sunburn and UV-induced skin damage will return to baseline immediately upon discontinuation.
> Important: Discuss all your medical conditions and any history of skin sensitivities with your healthcare provider before starting a new regimen containing Phenylbenzimidazole Sulfonic Acid.
There are no documented absolute drug-drug contraindications for the topical use of Phenylbenzimidazole Sulfonic Acid. However, it should not be applied simultaneously with other topical products that significantly alter the skin's pH to an extremely acidic level, as this may cause the sulfonic acid to precipitate out of the solution, rendering it ineffective.
There are no known interactions with food, including grapefruit, dairy, or high-fat meals, as the drug is not administered systemically.
Phenylbenzimidazole Sulfonic Acid does not interfere with standard blood or urine laboratory tests. It does not affect liver enzymes, creatinine, or blood glucose levels.
For each major interaction, explain:
> Important: Tell your doctor about ALL medications, supplements, and herbal products you are taking, even those for skin care.
There is a theoretical risk of cross-sensitivity for patients who have reacted to certain benzimidazole-based medications (such as the anthelmintic drug Thiabendazole). If you have had a reaction to these drugs, consult an allergist before using sunscreens containing Ensulizole.
> Important: Your healthcare provider will evaluate your complete medical history and skin type before recommending products containing Phenylbenzimidazole Sulfonic Acid.
FDA Pregnancy Category: Not Formally Assigned (Topical).
Data on the use of Phenylbenzimidazole Sulfonic Acid in pregnant women are limited. However, because systemic absorption is negligible when applied to intact skin, it is generally considered safe for use during pregnancy. The American College of Obstetricians and Gynecologists (ACOG) emphasizes the importance of sun protection during pregnancy to prevent melasma ('the mask of pregnancy'), which is exacerbated by UV exposure. If there are concerns about chemical filters, mineral-based sunscreens (Zinc Oxide/Titanium Dioxide) are recommended as alternatives.
It is not known whether Phenylbenzimidazole Sulfonic Acid is excreted in human milk. However, given the minimal systemic absorption from topical application, it is highly unlikely that clinically significant amounts would reach the infant through breast milk. Nursing mothers should avoid applying the product directly to the nipple or areola area to prevent the infant from ingesting the sunscreen during feeding.
As noted, use in children over 6 months is standard and encouraged for sun safety. Children's skin is thinner than adult skin, making it more susceptible to UV damage. While Phenylbenzimidazole Sulfonic Acid is safe, parents should monitor for signs of skin irritation. For children with highly sensitive skin or atopic dermatitis, a 'physical' sunscreen may be a better first-line choice.
In older adults, the skin's barrier function is often compromised due to age-related thinning (dermatoporosis). While Phenylbenzimidazole Sulfonic Acid is safe, elderly patients should be monitored for increased skin sensitivity. Additionally, because the elderly are at higher risk for Vitamin D deficiency, they should discuss with their doctor whether total sun avoidance requires Vitamin D supplementation.
No dosage adjustments or special precautions are required for patients with chronic kidney disease (CKD) or those on dialysis, as the drug does not enter the systemic circulation in significant quantities.
No dosage adjustments are required for patients with liver cirrhosis or hepatic failure.
> Important: Special populations, particularly pregnant women and those with chronic skin conditions, require individualized medical assessment by a healthcare professional.
Phenylbenzimidazole Sulfonic Acid (Ensulizole) is an organic UV-B filter. Its molecular structure consists of a benzimidazole ring substituted with a phenyl group and a sulfonic acid group. The sulfonic acid group is crucial as it allows the molecule to be neutralized with a base (like triethanolamine or sodium hydroxide) to become water-soluble.
Upon exposure to UV radiation, the molecule absorbs photons in the 280–320 nm range. The maximum absorption ($\lambda_{max}$) typically occurs around 302–306 nm. The energy from the UV light is absorbed by the $\pi$-electrons in the aromatic system, promoting them to an excited singlet state. The molecule then undergoes internal conversion to dissipate this energy as heat, preventing the UV rays from reaching the DNA of the keratinocytes in the epidermis.
The effect of Phenylbenzimidazole Sulfonic Acid is immediate upon the formation of a dry film on the skin. It does not have a systemic dose-response relationship; rather, its efficacy is determined by the concentration in the product and the thickness of the application. It is highly photostable, meaning it does not break down rapidly when exposed to sunlight, though it is often combined with other stabilizers to ensure long-lasting protection.
| Parameter | Value |
|---|---|
| Bioavailability | <1% (Topical) |
| Protein Binding | N/A (Negligible systemic levels) |
| Half-life | N/A (Not systemically active) |
| Tmax | N/A (Topical action) |
| Metabolism | Minimal (if any absorbed) |
| Excretion | Renal (trace amounts) |
Phenylbenzimidazole Sulfonic Acid is classified as an Organic UV-B Filter. It is often grouped with other benzimidazoles. It is chemically distinct from cinnamates (like Octinoxate) and salicylates (like Octisalate).
Medications containing this ingredient
Common questions about Phenylbenzimidazole Sulfonic Acid
Phenylbenzimidazole Sulfonic Acid, also known as Ensulizole, is primarily used as a UV-B filter in sunscreens and daily skin care products. Its main purpose is to absorb ultraviolet B radiation from the sun, which is the primary cause of sunburn and contributes to the development of skin cancer. Because it is water-soluble, it is a favorite ingredient for manufacturers creating 'oil-free' or lightweight sunscreens that do not feel greasy on the skin. It is frequently found in daily moisturizers, sports sunscreens, and makeup foundations with SPF. When used correctly, it helps prevent the immediate reddening of the skin and long-term damage caused by sun exposure.
The most common side effects are localized skin reactions, such as mild stinging, tingling, or redness at the site of application. These reactions are usually temporary and occur most often in individuals with sensitive skin or when the product is applied to dry, cracked skin. Another common issue is eye irritation; because the ingredient is water-soluble, it can run into the eyes with sweat, causing a sharp stinging sensation and temporary redness. Most users do not experience any systemic side effects because the chemical is not absorbed into the bloodstream in significant amounts. If you experience a persistent rash or severe itching, you should stop using the product and consult a healthcare provider.
Yes, there are no known medical interactions between the topical use of Phenylbenzimidazole Sulfonic Acid and the consumption of alcohol. Since the sunscreen filter remains on the surface of the skin and does not enter your systemic circulation, alcohol in your bloodstream will not affect how the sunscreen works. However, it is important to remember that alcohol can dehydrate the body and may make you less aware of the signs of heatstroke or overexposure to the sun. If you are spending time outdoors, it is always recommended to stay hydrated with water regardless of your sunscreen use. Always follow general safety guidelines for sun exposure and alcohol consumption.
Phenylbenzimidazole Sulfonic Acid is generally considered safe for use during pregnancy because it is applied topically and has negligible systemic absorption. Dermatologists often recommend consistent sunscreen use during pregnancy to help prevent melasma, a condition where hormonal changes cause dark patches to form on the skin when exposed to sunlight. While there are no specific studies showing harm to human fetuses, the low level of skin penetration makes it a low-risk option. If you have concerns about chemical sunscreens, you can always discuss using physical blockers like zinc oxide with your doctor. Always consult your obstetrician or dermatologist if you have specific concerns about skin care ingredients during pregnancy.
Phenylbenzimidazole Sulfonic Acid begins working as soon as it is applied to the skin, but it requires about 15 to 20 minutes to form a uniform, protective film. This 'setting time' is crucial because it ensures the filter is evenly distributed across the microscopic peaks and valleys of your skin surface. If you go directly into the sun or water immediately after application, the product may be wiped or washed away before it can provide full protection. For the best results, apply the product to dry skin indoors before heading outside. Remember that reapplication every two hours is necessary to maintain this protection throughout the day.
Yes, you can stop using products containing Phenylbenzimidazole Sulfonic Acid at any time without any risk of withdrawal symptoms or 'rebound' effects. Unlike some topical medications like steroids, sunscreen filters do not change the underlying physiology of your skin. However, as soon as you stop using the product, your skin will lose its protection against UV-B radiation. This means you will once again be at full risk for sunburn and long-term UV damage if you are exposed to the sun without an alternative form of protection. If you find that this specific ingredient irritates your skin, you should switch to a different UV filter immediately.
If you miss an application or forget to reapply your sunscreen, you should apply it as soon as you realize the omission. If you are already outdoors and have been in the sun unprotected for a significant amount of time, it is best to seek shade or go indoors immediately, as the damage from UV radiation happens quickly. Once you apply the missed dose, remember that it still takes about 15 minutes to set properly. A missed dose simply means your skin was vulnerable during the interval; it does not require you to apply 'double' the amount of product later. Consistency is the key to preventing skin cancer and premature aging.
No, Phenylbenzimidazole Sulfonic Acid does not cause weight gain. Weight gain is typically a side effect of systemic medications that affect metabolism, hormones, or appetite, such as oral steroids or certain antidepressants. Because Phenylbenzimidazole Sulfonic Acid is a topical sunscreen ingredient that stays on the surface of the skin, it does not enter the body in amounts that could influence your weight or metabolic processes. There is no clinical evidence or biological mechanism that would link the use of topical UV filters to changes in body weight. You can use this ingredient for sun protection without any concern regarding your weight.
Phenylbenzimidazole Sulfonic Acid can generally be used alongside most other medications. Since it is applied topically, it does not interfere with oral medications like blood pressure pills, antibiotics, or birth control. However, if you are using other topical treatments, such as acne creams (benzoyl peroxide) or anti-aging serums (retinol), it is best to apply those first and let them dry before applying your sunscreen. Some topical medications can make your skin more sensitive to the sun, making the use of a UV filter like Phenylbenzimidazole Sulfonic Acid even more important. Always inform your dermatologist about all the skin products and medications you are currently using.
Phenylbenzimidazole Sulfonic Acid is a chemical ingredient rather than a brand-name drug, so it is available in hundreds of different products from various manufacturers. You will find it listed in the 'Active Ingredients' section of the drug facts label on many store-brand and name-brand sunscreens. Because it is an over-the-counter (OTC) ingredient, you do not need a prescription to purchase products containing it. It is often listed by its other name, Ensulizole, on product labels. Whether you buy a premium brand or a generic store-brand sunscreen, the Phenylbenzimidazole Sulfonic Acid used is chemically identical and must meet the same FDA safety and efficacy standards.