Abstract Objective: To assess the impact of an intervention for teenage mothers with the involvement of maternal grandmothers on the prevalence of pacifier use in the first six months of life. Methods: This randomized clinical trial involved 323 teenage mothers, allocated to four groups: intervention with teenagers only, intervention with teenagers and their mothers, and respective controls. Six breastfeeding counseling sessions, including the recommendation to avoid the use of a pacifier, were delivered at the maternity ward and subsequently at the teenagers' homes, at seven, 15, 30, 60, and 120 days postpartum. Data on infant feeding and pacifier use were collected monthly by interviewers blinded to group allocation. The impact of the intervention was measured by comparing survival curves for pacifier use in the first six months of life and mean time to pacifier introduction. Results: The intervention had a significant impact on reducing pacifier use only in the group in which grandmothers were involved. In this group, the intervention delayed by 64 days the introduction of a pacifier (21-85 days), compared to 25 days in the group without the participation of grandmothers (65-90 days). Conclusions: The intervention reduced pacifier use in the first six months of life and delayed its introduction until beyond the first month when grandmothers were involved. The intervention did not have a significant impact when only teenage mothers were involved.