Can addiction be credibly invoked as an excuse for moral harms secondary to particular decisions to use drugs? This question raises two distinct sets of issues. First, there is the question of whether addiction is the sort of consideration that could, given suitable assumptions about the details of the case, excuse or mitigate moral blameworthiness. Most discussions of addiction and moral responsibility have focused on this question, and many have argued that addiction excuses. Here I articulate what I take to (...) be the best argument for this view, based on the substantial difficulty that people with severe addiction experience in controlling drug-related behavior. This, I argue, may in some cases be sufficient to ground a mitigating excuse, given the way in which addiction undermines agents’ responsiveness to relevant moral reasons to do otherwise. Much less attention has been devoted to a second set of issues that critically affect the possibility of applying this mitigating excuse in particular cases, derived from the ambivalent nature of agential control in addiction. In order to find a fitting response to moral harm, the person with the right standing to blame must make a judgment about the extent to which the agent possessed certain morally relevant capacities at the time of the act. In practice, this will often prove tremendously difficult to assess. The ethical challenge for the person with the right standing to blame is fundamentally one of making a judgment about matters that seem underdetermined by the available evidence. (shrink)