Metronidazole—Metronidazole is a bactericidal agent that disrupts nucleic acid synthesis with additional anti-inflammatory properties used in the treatment of rosacea. Allergic contact dermatitis to topical metronidazole has been reported.30-34 In 2006, Beutner at al35 patch tested 215 patients using metronidazole gel 1%, which revealed no positive reactions to indicate contact sensitization. Similarly, Jappe et al36 found no positive reactions to metronidazole 2% in petrolatum in their prospective analysis of 78 rosacea patients, further highlighting the exceptionally low incidence of ACD. Cross-reaction with isothiazolinone, which shares structurally similar properties to metronidazole, has been speculated.31,34 One patient developed an acute reaction to metronidazole gel 0.75% within 24 hours of application, suggesting that isothiazolinone may act as a sensitizer, though this relationship has not been proven.31
Neomycin—Neomycin blocks bacterial protein synthesis and is available in both prescription and over-the-counter (OTC) formulations. It commonly is used to treat and prevent superficial wound infections as an OTC antibiotic and also has otic, ophthalmologic, gastroenterologic, urologic, and peritoneal formulations. It also can be used in the dental and veterinary fields and is present in some animal feeds and in trace amounts in some vaccines for humans. Neomycin is a common antibiotic contact allergen, and the most recently reported 2017-2018 North American Contact Dermatitis Group data cycle placed it at number 12 with 5.4% positivity.37 Co-reactions with bacitracin can occur, substantially limiting OTC topical antibiotic options for allergic patients. A safe alternative for patients with neomycin (and bacitracin and polymyxin) contact allergy is prescription mupirocin.
Bacitracin—Bacitracin interferes with peptidoglycan and cell-wall synthesis to treat superficial cutaneous infections. Similar to neomycin, it also can be found in OTC antibiotic ointments as well as in antibacterial bandages. There are several case reports of patients with both type IV delayed hypersensitivity (contact dermatitis) and type I anaphylactic reactions to bacitracin38-40; patch testers should be aware of this rare association. Bacitracin was positive in 5.5% of patch tested patients in the 2017-2018 North American Contact Dermatitis Group data cycle,37 and as with neomycin, bacitracin also is commonly patch tested in most screening patch test series.
Polymyxin—Polymyxin is a polypeptide topical antibiotic that is used to treat superficial wound infections and can be used in combination with neomycin and/or bacitracin. Historically, it is a less common antibiotic allergen; however, it is now frequently included in comprehensive patch test series, as the frequency of positive reactions seems to be increasing, probably due to polysensitization with neomycin and bacitracin.
Nystatin—Nystatin is an antifungal that binds to ergosterol and disrupts the cell wall. Cases exist of ACD to topical nystatin as well as systemic ACD from oral exposure, though both are quite rare. Authors have surmised that the overall low rates of ACD may be due to poor skin absorption of nystatin, which also can confound patch testing.41,42 For patients with suspected ACD to nystatin, repeat open application testing also can be performed to confirm allergy.