BACKGROUND Indirect evidence suggests that using waiting list control designs in behavioural research may have unintended consequences. The aim of this study was to estimate the effects of a waiting list design on alcohol consumption among individuals who had looked online for help.

METHODS A two-arm randomised controlled trial was employed. The intervention group was informed that they belonged to the intervention group and would receive immediate access to a digital alcohol intervention. The waiting list control group was informed that they belonged to the group that had to wait four weeks to be given access to the intervention and in the meantime, they would be given a summary of their drinking. However, both groups received immediate access to the same digital alcohol intervention; the experimental contrast was thus between being told to wait or not.

RESULTS We randomised 3388 participants (intervention: 1692, waiting list: 1696). Data were available for 954 participants at 1-month follow-up. We found no strong evidence that alcohol consumption differed between groups, but the evidence pointed towards the intervention group reporting lowering weekly alcohol consumption compared to the waiting list control group (IRR = 0.95, 95 % CI = 0.83; 1.08, probability of effect = 78.8 %).

CONCLUSION We found no strong evidence that being informed that access to an intervention would be delayed produced differential self-reported alcohol consumption compared to being informed that access would be immediate. We did find a difference in engagement with the intervention materials, indicating that the experimental manipulation was successful.