There is a growing interest in veterinary medicine toward the use of alternative treatments including nutraceuticals. This open-label trial evaluated the use of the nutraceutical l-theanine (Anxitane, Virbac Animal Health, Fort Worth, TX) (N-ethyl-l-glutamine) in client-owned dogs with a history of storm sensitivity. Dogs had to be between 1 and 8 years and were recruited from general practices in the study area. All enrolled dogs were healthy based on physical examination and laboratory analysis and not currently being treated for any chronic medical or behavioral disorder. Owners completed a questionnaire for the initial storm. Then, after starting the test article, owners completed the same questionnaire for each of 5 subsequent storms. Questionnaires evaluated 11 individual behavior manifestations of storm sensitivity using a 0-5 Likert scale. Storm severity was also rated on a 0-5 Likert scale. At the end of each storm, owners evaluated time to return to the patient's normal baseline behavioral state. Owners were given a standardized protocol of environment and behavior management, but no other behavior modification was prescribed. Eighteen dogs completed the trial and were available for statistical analysis. There was a statistically significant decrease (P < 0.0001) in global anxiety scores from baseline to exit evaluation. Additionally, time to return to baseline normal scores demonstrated a significant decrease (P = 0.0063) from baseline storm to last storm evaluated. Treatment success was achieved for the behaviors of drooling (83.33%), following people (75%), panting (76.47%), pacing (78.57%), and hiding (78.57%). Owner satisfaction with treatment was 94% (17 of 18). This study suggests that l-theanine can be an effective treatment for storm sensitivity, decreasing severity of the dog's overall response, time for the dog to return to baseline after storm ends, and diminishing drooling, following people, pacing, panting, and hiding.