Background: Childhood allergy is an abnormal sensitivity or reaction of the immune system to a substance (allergen) that we eat, inhale, or touch. Allergens can affect anyone regardless of age, gender, race, or socioeconomic status. Objectives: To assess existing knowledge on allergens among adolescents, to determine effectiveness of teaching program about allergens among the adolescents, and to associate the post knowledge with selected sociodemographic variables of adolescents. Methodology: A quasi-experimental research design (one group pre- and post-test design) was adopted for the study. The population of the present study was 400 adolescents studying 9th and 10th standard in Seventh Day Adventist Higher Secondary School, Puducherry. A hundred samples were chosen using the lottery method. Then, the sociodemographic variables of adolescents and a tool that consists of 25 items of dichotomous tool to assess the knowledge on allergens’ data were collected. Results: Out of 100 adolescents, 10 (10%) had inadequate knowledge, 12 (12%) adolescents had moderately adequate knowledge, and five 78 (78%) had adequate knowledge regarding allergens after the teaching program in the post-test. Conclusion: There was significant association between the occupation of the father and the post-test knowledge at p > 0.05 level, and there was no significant association between sociodemographic variables and post-test knowledge level of the adolescents on allergens.