If there is a true benefit from dextromethorphan, it suggests that some central component to pruritus or at least to the repetitive behaviors that clients and veterinarians interpret as pruritus (e.g. licking, biting) exists. Opioid antagonists have been successfully used in other species to treat repetitive behaviors.^3,4 For example, dextromethorphan has been effective in treating crib-biting in horses.⁵ It is hypothesized that dextromethorphan works by N-methyl-D-aspartate blockade and not by opioid blockade. The exact mechanism of action is unknown, but N-methyl-D-aspartate receptors are found in the spinal cord and brain and are involved in learning and memory, tolerance, and sensitization.

When new findings regarding a drug's antipruritic effects are reported, it is tempting to prescribe it as a first-choice drug; however, newer is not always better or appropriate. The long-term safety of dextromethorphan is unknown, and the dogs in this study received the drug for only two weeks. Clients should be aware that this drug is not free of side effects, which include sedation, vomiting, and diarrhea.

Treating dogs with chronic atopy will always require vigilant monitoring for breakthrough infections and secondary parasitic infestations. Traditional therapies for pruritus (immunotherapy, antihistamines, glucocorticoids, and cyclosporine) are indicated initially. Dextromethorphan may be useful in patients with no obvious signs of inflammation or in cases of a suspected repetitive behavior. But it is important to note that no studies are available documenting the existence of habitual pathogenesis. Perhaps there is a place for a drug such as dextromethorphan in the treatment of atopy, but for now my recommendation is to focus on traditional antipruritic therapies and monitoring of underlying disease.

1. Dodman NH, Shuster L, Nesbitt G, et al. The use of dextromethorphan to treat repetitive self-directed scratching, biting, or chewing in dogs with allergic dermatitis. J Vet Pharmacol Ther 2004;27:99-104.

2. Kukanich B, Papich MG. Plasma profile and pharmacokinetics of dextromethorphan after intravenous and oral administration in healthy dogs. J Vet Pharmacol Ther 2004;27:337-341.

3. Dodman NH, Shuster L, Court MH, et al. Investigation into the use of narcotic antagonists in the treatment of a stereotypic behavior pattern (crib-biting) in the horse. Am J Vet Res 1987;48:311-319.

4. Kenny DE. Use of naltrexone for treatment of psychogenically induced dermatoses in five zoo animals. J Am Vet Med Assoc 1994;205:1021-1023.

5. Rendon RA, Shuster L, Dodman NH. The effect of the NMDA receptor blocker, dextromethorphan, on cribbing in horses. Pharmacol Biochem Behav 2001;68:49-51.

"Dermatology Update" was contributed by Karen A. Moriello, DVM, DACVD, Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53706.