Objective: To assess the irritation and sensitization potential of ibuprofen medication. Methods: This study had two stages. Stage 1 was to evaluate the irritant potential of continuous application of the plaster, and the challenge phase to assess contact sensitivity (allergy). The cumulative irritancy potential was evaluated using an adaptation of the Shelanski method. Healthy adults (≥18 years of age) (n=210) were treated simultaneously with one ibuprofen medicated and one placebo plaster applied in a randomized fashion to either the left or right side of the lower back. During the stage 1, plasters were applied on Days 1, 3, 5, 8, 10, 12, 15, 17, and 19 and the final plaster removed on Day 22. At each scheduled visit plasters and applications sites were assessed for degree of adhesion and skin irritation (score of 0=no irritation to 7=strong reaction spreading beyond test sites), respectively. The stage 2 followed a two-week washout period. A plaster was applied on Day 36 for 48 hours and assessment occurred on Days 38, 39, and 40. Results: The mean cumulative irritation score during the stage 1 was lower for the ibuprofen medicated plaster than the placebo plaster (0.32 vs 1.23, respectively). Three (1.4%) subjects experience a dermal reaction of grade ≥3 for the ibuprofen medicated plaster compared with 27 (12.7%) for the placebo plaster. Following stage 2, 12 subjects (6.2%) with the ibuprofen medicated plaster and four (2.2%) with the placebo plaster had skin reaction of assessment grade higher than that in stage 1. One subject for the ibuprofen and two for the placebo plaster had reactions with grade >2. Conclusion: The findings indicate that the both the irritancy and contact sensitization of the ibuprofen medicated plaster is acceptable.