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Patch test reactions in a patient with purpuric contact dermatitis to 5% benzoyl peroxide are described. Alterations of the capillary endothelium included obliteration of the lumina with perivascular mononuclear cell infiltrates. There were no epidermal alterations. These observations are discussed with reference to the data in the literature on vascular damage in "dermal" contact dermatitis.

The incidence of contact allergy to a 5% benzoyl peroxide (BP) gel has been studied by patch testing in 222 patients. Positive allergic reactions to BP were found in 3.1% of cases in a general clinical population (94 patients), whereas 5.8% of 69 patients with eczematous dermatoses were found to be BP-sensitized. In a group of acne vulgaris patients having used a commercial 5% or 10% BP gel for a period of 12 days to 3 1/2 years (mean: 10.7 months), positive allergic reactions were found in three cases (5.1%).

Four formulations of benzoyl peroxide, two 5% and two 10% gels, were assayed for contact sensitizing capability by the human maximization test. Seventy-six per cent became sensitized. Sensitized subjects reacted to all four formulations. Benzoic acid and related compounds failed to elicit an allergic reaction.

A subthreshold dose of 0.1% DMBA in oil applied to hamster buccal pouch three times a week for 10 weeks did not result in the development of carcinoma in up to 25 weeks from the start of the experiment. However, if the subthreshold dose of DMBA was followed by 6 weeks of no treatment and six weeks of 40% benzoyl peroxide treatment, carcinomas developed rapidly. Both benzoyl peroxide and its acetone solvent are noncarcinogenic. An experiment carried out on sixty-six young adult male and female Syrian hamsters demonstrated these results, using appropriate controls.

After four months of use, a 13-year-old girl with acne developed allergic contact urticaria to 5 percent topical benzoyl peroxide. This was confirmed by closed patch tests. It is suggested that concomitant use of an abrasive scrub cleanser may have damaged the stratum corneum and initiated or enhanced sensitization.

Of 739 dermatitis patients, 4.1% showed a positive patch test reaction to benzoyl peroxide 1% in petrolatum. Forty-one patients with leg ulcers (patch test: negative to benzoyl peroxide) were treated with a 20% benzoyl peroxide emulsion for 4 weeks. After this therapy, the sensitization rate was as high as that for the maximization test, namely 76%. In most patients, the post-therapeutic allergy to benzoyl peroxide was accompanied by contact allergies to other therapeutic agents applied. Only 9 of 31 sensitized patients showed a monovalent benzoyl peroxide allergy. From the allergologic point of view, benzoyl peroxide cannot be recommended for the treatment of leg ulcers. However, there appears to be no danger in the treatment of acne vulgaris.

A case-control study utilizing available medical records was carried out to investigate possible links between use of topical benzoyl peroxide for acne, and malignant melanoma. One hundred and fifty nine cases and 213 matched controls were used in the analysis which shows no significant association between the use of benzoyl peroxide and the occurrence of malignant melanoma, nor between the occurrence of acne and malignant melanoma.

Benzoyl peroxide patch testing was performed on 60 young adults who had participated in a double-blind acne study. 25% of those who had used a benzoyl peroxide product showed reactivity of +1 or greater (on a 0--3 scale). None of these individuals demonstrated symptoms suggestive of irritancy or allergy greater than those who did not have positive patch tests and who were on the same treatment program. The positive patch tests did not correlate with acne treatment responses. 2 of 44 individuals in the acne study group developed clinical allergy and had dramatically positive patch tests, while the majority of individuals with positive patch tests could use products containing benzoyl peroxide daily without significant adverse effects.

It is reported that a 62-year-old man with total hip prosthesis developed a recurrent sterile fistula and loosening of his prosthesis due to allergy to the bone cement component benzoyl peroxide.

Sixteen patients with chronic leg ulcers were treated with 10% benzoyl peroxide gel for 6 weeks. At the end of the treatment, patch tests with 2% benzoyl peroxide in petrolatum and 10% in a gel showed that nine patients (56%) had become sensitized. This high sensitization rate is in good accord with the results obtained by experimental investigations using human maximization tests and repeated insult patch tests. A strong sensitizer such as benzoyl peroxide should not be used in the routine treatment of chronic leg ulcers, despite its good healing effect.

In order to conform to the conditions in which a 5% benzoyl peroxide compound is being used, a new methodology has been developed and applied to 24 volunteers: photobiologic tests were performed and UVB irradiation at infra- and supra-MED times of exposure as well as UVA irradiation on usually covered and usually uncovered areas, after single and multiple applications. This double-blind study (vehicle alone and active substance alone) has been carried out in each subject. UVB phototoxicity resulted in the occurrence of an erythema for an infra-MED time of exposure and/or in changes in the erythema as compared to an adjacent control area for the supra-MED time of exposure. This was found in 8 patients (33%), only on repeatedly irradiated areas, but never with the vehicle alone. No UVA phototoxicity was evidenced. This study confirms the possible phototoxic action of benzoyl peroxide after UVB exposure. It emphasizes the usefulness of repeating the applications on both covered and uncovered areas in photosensitivity tests of a substance.

The predictive phototoxicity test known as photoepidermotest consists of twice daily applications of the product tested for 16 days, followed by total spectrum irradiation with a 2,500-W Xenon lamp in infraerythema and supraerythema doses. Phototoxicity is revealed by an erythema with the infraerythema dose and/or by accentuation of the erythema provoked by the supraerythema dose when compared with the "irradiation only" (i.e., no product) control skin area. This technique, used in a controlled, double-blind trial, showed that a 10% benzoyl peroxide (BPO) gel was phototoxic in eight out of 18 subjects tested (44%). It also showed that adding a UVB filter (phenylbenzimidazole sulfonic acid 1%) to the BPO preparation was effective in preventing phototoxic reactions in seven of these eight subjects.

The investigation of patients with leg ulcers, treated with a concentrated benzoylperoxide solution, revealed a sensitization to the substance in nearly half of the probands. Therefore and because of the probable promotion of neoplastic epidermal proliferation, benzoylperoxide-containing solutions should be used for the treatment of leg ulcers in selected cases only.