Internet-delivered psychological treatment as an add-on to treatment as usual for common mental disorders: A systematic review with meta-analysis of randomized trials

•There is a demand for effective psychological treatments in primary care.•Studies have recently evaluated benefits of adding internet-based CBT to usual care.•There may be incremental effects on depression in adults.•More information about treatment as usual would increase the value of studies.•There are important knowledge gaps, particularly for children and adolescents.