CONTACT DERMATITIS 2008: 59: 115–132 * COPYRIGHT ª BLACKWELL MUNKSGAARD 2008 * ALL RIGHTS RESERVED * CONTRIBUTIONS TO THIS SECTION WILL NOT UNDERGO PEER REVIEW, BUT WILL BE REVIEWED BY THE EDITOR * Concomitant allergic contact dermatitis from diphenhydramine and metronidazole Contact Dermatitis 2008: 59: 115–116 Beatriz Ferna ´ndez-Jorge, Jaime Goday Buja ´n, Rosa Ferna ´ndez-Torres, Romina Rodrı ´guez-Lojo and Eduardo Fonseca Department of Dermatology, Hospital Juan Canalejo, C.P. 15001 A Corun ˜a, Spain Key words: allergic contact dermatitis; anti- histamine; diphenhydramine; medicaments; metronidazole; patch test. Case Report A 45-year-old woman had a 15-day his- tory of an acute, itchy, vesicular and erythematosus eruption around the mouth (Fig. 1). She had previous rosa- cea that had been treated with an anti- allergic cream, a medication composed of chlorophyll, kamillosan, erythromy- cin, metronidazole and diphenhydra- mine, the concentrations of which were not recorded. The lesions cleared entirely within 10 days of discontinu- ing the antiallergic cream and applying a topical corticosteroid. Patch testing according to ICDRG criteria was nega- tive to the Spanish Contact Derma- titis Investigation Group (GEIDC) baseline series, the only positive being nickel (þþ at D4). Patch testing with the anti-allergic cream was positive both at D2 (þþ) and D4 (þþþ). Photo-patch tests with the anti-allergic cream were not performed. Photo- patch test with the antiallergic cream was not performed. We subsequently performed patch tests with all the ingredients of the cream, and readings were performed after two days of occlusion and positive reactions were noted to diphenhydramine (þþ at D2 and D4) and metronidazole (þþ at D4). A photopatch test was carried out after ultraviolet A irradiation (5 J/cm 2 ) and showed a positive reac- tion to diphenhydramine at 3 D (þþ) with similar strength to previous posi- tive patch test. Results are detailed in Table 1. Controls were performed in 12 healthy patients with negative results. Discussion Metronidazole is an imidazole deri- vative used against parasites and an- aerobic bacteria which seldom causes allergic contact dermatitis (1–4). Di- phenhydramine hydrochloride is an H 1 receptor antagonist, which rarely can trigger allergic and/or photoaller- gic contact dermatitis (5–9). Photo- allergic contact dermatitis caused by diphenhydramine hydrochloride is rare, and only 4 cases have been reported in the English medical lit- erature (5–7), and only 1 case of the coexistence of both allergic and photo- allergic contact dermatitis caused by diphenhydramine has been previously described (6). In our case, the reactions to the patch test and the photopatch test were equally positive; so, photoal- lergy or photoagravation from diphen- hydramine were not considered likely. We believe that our patient suffered from a concomitant allergic contact dermatitis because of metronidazole and diphenhydramine, both contained in the same medication, which has not formerly been reported. Cross-reac- tions between diphenhydramine and metronidazole have not been described and are highly unlikely, given that their chemical structures are different. Fig. 1. Eczematous reaction around the mouth. Table 1. Results of epicutaneous studies Substance Patch test Photopatch test 2D 4D 1D 3D Standard series: nickel  þþ NT NT Antiallergic cream ‘as is’ þþ þþþ NT NT Cream components Diphenhydramine 1% pet. þþ þþ  þþ Metronidazole 5% pet. þþ þþ   Metronidazole 10% pet. þþ þþ   Remaining ingredients     NT, not tested.