Study objective Corticosteroids (steroids) are often used to mitigate symptoms and prevent subsequent reactions in emergency department (ED) patients with allergic reactions, despite a lack of evidence to support their use. We sought to determine the association of steroid administration with improved clinical outcomes. Methods Adult allergy-related encounters to 2 urban EDs during a 5-year period were identified and classified as “anaphylaxis” or “allergic reaction.” Regional and provincial databases identified subsequent ED visits or deaths within a 7-day period. The primary outcome was allergy-related ED revisits in the steroid- and nonsteroid-exposed groups, adjusting for potential confounders with a propensity score analysis; secondary outcomes included the number of clinically important biphasic reactions and deaths. Results Two thousand seven hundred one encounters (473 anaphylactic) were included; 48% were treated with steroids. Allergy-related ED revisits occurred in 5.8% and 6.7% of patients treated with and without steroids, respectively (adjusted odds ratio OR 0.91; 95% confidence interval CI 0.64 to 1.28), with a number needed to treat (NNT) to benefit of 176 (95% CI NNT to benefit 39 to ∞ to NNT to harm 65). The adjusted OR in the anaphylaxis subgroup was 1.12 (95% CI 0.41 to 3.27). In the allergic reaction group, the adjusted OR was 0.91 (95% CI 0.63 to 1.31), with an NNT to benefit of 173 (95% CI NNT to benefit 38 to ∞ to NNT to harm 58). In the steroid and nonsteroid groups, there were 4 and 1 clinically important biphasic reactions, respectively. There were no deaths. Conclusion Among ED patients with allergic reactions or anaphylaxis, corticosteroid use was not associated with decreased relapses to additional care within 7 days.