BackgroundThe antidepressant effect of repetitive transcranial magnetic stimulation (rTMS) is partly placebo, making blinding integrity important. Blinding of high-frequency rTMS and intermittent theta burst stimulation (iTBS) has been reported as successful at study end. However, blinding integrity at study start is rarely reported. The aim of this study was to investigate blinding integrity during a treatment course of iTBS over the dorsomedial prefrontal cortex (DMPFC) in depression.MethodsForty-nine patients with depression from a double-blind-designed randomized controlled trial (NCT02905604) were included. Patients received either active or sham iTBS over the DMPFC with a placebo coil. The sham group received iTBS-synchronized transcutaneous electrical nerve stimulation.ResultsAfter one session, 74% of participants were able to correctly guess their treatment allocation. This was above chance level (p = 0.001). The percentage dropped to 64% and 56% after the fifth and last sessions. Belonging to the active group influenced the choice to guess “active” (odds ratio: 11.7, 95% CI 2.5–53.7). A higher treatment intensity of the sham treatment increased the probability to guess “active”, but pain did not influence the choice.ConclusionsBlinding integrity in iTBS trials must be investigated at study start to avoid uncontrolled confounding. Better sham methods are needed.