Many consumer behaviors are both addictive and social. Understanding how these two phenomena interact informs basic models of human behavior, and matters for policymakers when the behavior is regulated. I develop a new model of demand that incorporates both addiction and social interactions and show that, under certain conditions, social interactions reinforce the eﬀects of addiction. I also show how the dynamics introduced by addiction can solve the pernicious problem of identifying the causal eﬀects of social interactions. I then use the model to illustrate a new and important identiﬁcation problem for studies of social interactions: existing estimates cannot be used to draw welfare conclusions or even to deduce whether social interactions increase aggregate demand. Finally, I develop a method that allows researchers to distinguish between two common forms of social interactions and draw welfare conclusions.