Abstract
BACKGROUND: Cognitive behavioural therapy (CBT) is an effective intervention for adults with attention-deficit/hyperactivity disorder (ADHD). While both brief (6-session) and standard (12-session) formats show efficacy, it remains unclear whether treatment response differs between distinct ADHD clinical presentations. METHODS: This secondary analysis of a randomized trial included 80 adults with ADHD (57.5% male; mean age = 41.26 ± 9.32 years) allocated to brief or standard CBT. Clinical and functional outcomes were assessed at baseline, posttreatment, as well as 3- and 6-month follow-ups. Linear mixed-effects models and the Reliable Change Index (RCI) evaluated interactions between time, treatment format and ADHD presentation (inattentive vs. combined). RESULTS: CBT yielded significant improvements across all domains, with 94.5% of participants achieving reliable symptomatic improvement. A significant time × presentation interaction (p = 0.001, ηp(2) = 0.135) revealed a steeper core symptom reduction in the combined group, which survived sensitivity analyses controlling for baseline severity. Regarding format, the brief version produced greater observer-rated symptom improvement, whereas the standard format yielded greater long-term functional gains (p = 0.009, ηp(2) = 0.061). Improvements were sustained at the 6-month follow-up. No robust three-way interactions emerged. CONCLUSIONS: CBT is highly effective and durable across ADHD presentations and formats. Although the combined presentation exhibited a more pronounced reduction in core symptoms, this differential trajectory represents a promising clinical trend requiring cautious interpretation. While brief formats efficiently address core symptoms, standard programs may better optimize long-term functional recovery.