Background: Cognitive flexibility may serve as a protective factor against addiction readiness and the harms resulting from parental substance use among adolescent girls; therefore, identifying effective interventions to reduce these harms is of particular importance. Few studies have examined the efficacy of transdiagnostic treatment in this context.

Aims: The present study aimed to investigate the effectiveness of Unified transdiagnostic treatment on addiction readiness and cognitive flexibility in female adolescent with drug-abusing parents.

Method: This experimental study was conducted with a pretest-posttest design with a control group. The statistical population included all girls aged 13 to 17 with drug-abusing parents in Mashhad (2023). Thirty participants were purposefully selected and randomly assigned to two groups: experimental (15 participants) and control (15 participants).The research instruments included the Iranian Addiction Potential Scale (Zargar, 2006) and the Cognitive Flexibility Inventory (Dennis and Vander Wal, 2010). The experimental group received the Unified Protocol intervention in 8 weekly 90-minute sessions. The data were analyzed using Multivariate Analysis of Covariance (MANCOVA) with SPSS-21.

Results: The findings indicated that after controlling for pretest scores, there was a significant difference between the two groups on the dependent variables. Unified transdiagnostic treatment significantly reduced Active readiness (F_{(1, 26)} = 22.50, p = 0.001), Passive readiness (F_{(1, 26)} = 4.71, p = 0.001), addiction readiness (F_{(1, 26)} = 58.83, p = 0.001) and significantly increased Problem-solving processing (F_{(1, 26)} = 34.17, p = 0.001), Perception of controllability (F_{(1, 26)} = 25.81, p = 0.001) and cognitive flexibility (F_{(1, 26)} = 68.53, p = 0.001) in the experimental group.

Conclusion: Unified transdiagnostic treatment, by focusing on common psychological processes, can effectively reduce addiction readiness and enhance cognitive flexibility in adolescents; therefore, this approach can be used as an applied intervention in preventing addiction in at-risk adolescents.