As part of a process evaluation, we explored in semi-structured interviews the experiences of 19 mothers who had taken part in a trial to reduce infant formula-milk intake and promote healthy weight gain, and reflections of three facilitators who delivered the intervention and control group protocols. Mothers appreciated the nonjudgmental support provided during the trial, after experiencing stigma and receiving limited advice on how, how much, and how often formula-milk should be given. The information mothers had previously found, printed on formula-milk tins, or provided by family, friends, and health professionals was often perceived as contradictory; the trial guidance also conflicted with social norms relating infant health positively with weight gain. For those recruited into the trial with older infants, who had already exceeded the recommendations, reducing formula-milk amounts was difficult. The findings highlight the difficulties of addressing a highly stigmatized, complex social practice with an individual, behavioral intervention approach.