Background: Cosmetic products are widely used among teenage girls. Inadequate knowledge regarding their adverse effects may lead to health problems such as allergies, dermatitis, acne, and skin irritation

Objectives: 1.To assess the knowledge regarding adverse effects of cosmetic usage among teenage girls.

2. To evaluate the effectiveness of a Structured Teaching Programme (STP).3.To determine the association between knowledge and selected demographic variables.

Methods: Aquantitative pre-experimental one-group pretest-posttest design was adopted among 60 teenage girls aged 14–17 years selected through simple random sampling. A structured questionnaire consisting of 30 items was used to assess knowledge.

Results: The mean pretest knowledge score was 14.62, whereas the mean posttest score increased to 20.2. The paired t-test value was 14, indicating significant improvement in knowledge following the Structured Teaching Programme.

Conclusion: The Structured Teaching Programme was effective in improving knowledge regarding adverse effects of cosmetic usage among teenage girls.