Does allocation to a control condition in a Randomized Controlled Trial affect the routine care foster parents receive?

•We examine treatment as usual in an RCT control group in foster care.•Being part of a control group affects treatment as usual for foster parents.•Being part of a control group impacts counseling frequency and help-seeking behavior.

‘Strengthening Foster parents in Parenting’ (SFP) is a support program for foster parents who care for foster children with externalizing problem behavior. Its effectiveness was examined with a Randomized Controlled Trial (RCT). In this paper, we examine the treatment as usual (TAU) that was offered in the control condition of this RCT. For this purpose, the TAU from the SFP control group was compared with TAU provided to a similar group of foster parents outside any RCT.Our results show that TAU is diverse and varies widely. Furthermore, being part of the control condition was positively associated with both the counseling frequency from the foster care services and with external help-seeking behavior (finding and using additional support).In order to prevent condition contamination in future trials, TAU should be clearly described and standardized, and treatment fidelity should be carefully monitored.