Phototoxicity and Photoallergy
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It has been known for decades that some topical products can become safety concerns when exposed to UV light. These products contain compounds that (a) absorb UV or visible radiation and (b) become reactive, damaging vital skin structures. There are 2 general classes of photo-reactive compounds, namely phototoxins (PT) and photoallergens (PA). The illustration below highlights the key features of each pathway.
Key Differences Between Phototoxicity and Photoallergy
While both phototoxicity and photoallergy are forms of light-induced skin reactions, their biological mechanisms and clinical outcomes differ significantly.
Phototoxicity
Phototoxicity is caused by direct cellular damage from reactive oxygen species or photoproducts generated when a chemical absorbs UV(A) or visible light. The reaction is immediate, usually occurring within minutes to hours, and the severity is strongly dependent on both the dose of the substance and the intensity of light exposure, as is typical for direct toxic effects. Compounds that are known to cause phototoxic reactions include psoralens, citrus oils and coal tar.
Photoallergy
In contrast, photoallergy is an immune-mediated delayed hypersensitivity reaction. It requires prior sensitization, meaning the immune system must first recognize the compound or its light-modified form as a foreign hapten. Once sensitized, even minimal amounts can trigger a response, typically appearing 24 to 72 hours after exposure. Clinically, phototoxic reactions resemble sunburn, characterized by redness and blistering, whereas photoallergic reactions resemble eczema, presenting as itchy, inflamed dermatitis that may spread beyond the exposed area. Compounds that are known to cause photoallergic reactions include sunscreen actives, some fragrance oils and NSAIDS.
In summary, and in accordance with FDA S10 and ICH guidelines, when a drug or cosmetic ingredient exhibits absorption in the UV/visible range, or when existing literature indicates photosensitivity, or when the formulation utilizes known sensitizers, testing should be conducted to confirm that the product is safe. This page is dedicated to our clinical capabilities with respect to PT/PA, but there are pre-clinical methods available which are very useful for screening new compounds/formulas before you commit to a human test.
Method for Assessing the Phototoxic Potential of a Topical Product
CPT℠ follows the methods of Willis and Kligman (J Invest Dermatol 1968;51:116-119) for both phototoxicity and photoallergy.
Our phototoxicity method is a 4-day procedure where the erythema/redness of UV-exposed sites is compared to erythema/redness of unexposed sites. If the site that is UV-exposed shows a greater response, then the product is deemed a phototoxicity risk.
Method for Assessing the Photoallergic Potential of a Topical Product
The photoallergy method is a 5-6 week trial that is essentially the fusion of an HRIPT with UV exposure. As with the phototoxicity method, a positive result is where the allergic response in the UV-exposed sites are compared to the untreated sites, as seen below.
Determination of photoallergenicity potential:
| Test Condition | Conclusion | |
| Treated | Treated | |
| Non-Irradiated | Irradiated | |
| (positive) | (positive/negative) | Possible contact allergen |
| (negative) | (positive) | Possible contact photoallergen |
| (negative) | (negative) | Not a contact allergen or photoallergen |
Why is CPT℠ the Best Lab for Your Photosafety Assessments
With decades of experience in photobiology and clinical safety testing, CPT℠ Labs is uniquely positioned to support your R&D, formulation, and compliance efforts.
From in vitro phototoxicity assays to clinical photoallergy evaluations, our team designs comprehensive, regulatory-aligned programs that help bring safe, effective products to market.
If you’re ready to take the next step in your photosafety strategy, contact us today to discuss how we can tailor testing solutions to your specific needs.