Is benzoyl peroxide compatible with sun exposure?

The benzoyl peroxide is a particularly effective agent for treating acne as it inhibits the bacteria responsible for skin inflammation. However, it has a molecular structure with chromophores, that is, a group of atoms containing one or more double bonds. Now, when a chromophore is exposed to light, its electrons get excited and absorb specific wavelengths, which elevates them to a higher energy level. This makes the molecule more reactive. It then interacts with the surrounding oxygen and, through molecular dissociation, induces the generation of ROS which are involved in the phototoxicity reaction. Some research indicates that benzoyl peroxide could be phototoxic due to its chromophores.

• In order to highlight the phototoxic potential of benzoyl peroxide, a study was conducted on 24 volunteers, using a product containing 5% benzoyl peroxide. Photobiological tests and irradiations with UVA and UVB rays were carried out. The results showed that 33% of the subjects presented with erythema, even though the UVB irradiation was less than the minimal erythemal dose (MED), suggesting phototoxicity in response to UVB. Conversely, no phototoxicity associated with UVA was observed in this analysis.

• A few years later, the same researchers conducted another study over 16 days, this time using a 10% benzoyl peroxide gel via a photopatch test (photoepidermotest). The results showed phototoxicity in 44% of the participants (8 out of 18), due to the appearance of erythema on the treated areas compared to the control areas with irradiation less than the MED.

• In 2004, a study published by the National Center for Biotechnology Information examined the impact of benzoyl peroxide gel and other substances on UVB-induced erythema. Out of 30 volunteers, the MED with or without the application of benzoyl peroxide was the same, suggesting a lack of influence on the risk of burning.

The conclusions of these three studies must be approached with caution due to the limited number of volunteers and the discrepancies between the study results, which do not allow for an accurate assessment of the risk frequency or definitive conclusions about the photosensitising power of benzoyl peroxide.

Beyond this phototoxic action, the benzoyl peroxide is known for its exfoliating action, which allows it to remove dead skin cells and promote cellular renewal. However, exfoliation can increase skin's sensitivity to light because, when the top layer of skin is removed, it becomes more exposed and less protected against the harmful effects of UV rays. This could intensify the photosensitising reaction if benzoyl peroxide is used at the same time as exposure to light. This is, however, a hypothesis, as no study has shown a direct action of the exfoliating property of benzoyl peroxide on its photosensitive character.

The possibility that benzoyl peroxide may be photosensitising, and particularly phototoxic, cannot be ruled out. As a precautionary principle, it is therefore advised to apply it in the evening and to use sun protection the following morning.

Note : Any treatment or chemical potentially responsible for a photosensitive reaction should be discontinued. In many cases, the skin reaction gradually subsides after the cessation of treatment, but this can vary from person to person depending on the severity of the reaction and individual sensitivity. Therefore, it is strongly recommended to consult a doctor at the first signs of symptoms. In some cases, topical corticosteroids may be prescribed to reduce inflammation and soothe the skin.