A randomized factorial trial of internet-delivered cognitive behavioural therapy: An 8-week program with or without extended support and booster lesson.

While internet-delivered cognitive behavioural therapy (ICBT) is effective, some patients suggest extended support post-treatment could improve care. In this randomized factorial trial, we examined the benefits of an 8-week therapist-assisted ICBT program offered with or without an optional 4-week extension of support (Factor 1) and with or without an optional booster lesson (Factor 2). Patients screened for ICBT for depression and/or anxiety were randomly assigned to the conditions ( = 434) and we examined the use of the extension and booster, differences between those who did or did not use extension or booster, and the impact of the extension or booster on outcomes, engagement, and satisfaction at 26-weeks post-enrollment. Therapists recorded time and observations with offering support during the extension and booster. In the extension group, 54.4% ( = 56) requested the extension, while in the booster group 50.9% ( = 56) accessed the booster, and in the combined group, 41.6% ( = 47) requested the extension and 51.3% ( = 58) accessed the booster. Those who requested the extension were older, and more likely to report medication and mental health service use and severe mental health-related disability at pre-treatment; they also reported putting less effort into ICBT and finding skills more difficult. The booster was more often used among those with lower symptom severity, and those who put more effort into and had more positive experiences with ICBT. As expected, those assigned to extension sent more messages to their therapist, and those assigned to booster logged in more often. Therapists also took more time to deliver ICBT with an extension (>18 min) or booster (>13 min) compared to the 8-week program, and perceived extension and booster as beneficial for some, but not all patients. Treatment satisfaction was high across conditions, and effect sizes were large from pre-treatment to 26-week follow-up on most measures. No significant group differences were found in this study. Lack of group differences, however, could reflect low use of the extension and booster. Results provide helpful information about the demand for extensions and boosters, and provide directions for future research.