We examine the ethical, social and regulatory barriers that may hamper research on therapeutic potential of certain controversial controlled substances like marijuana, heroin or ketamine. Hazards for individuals and society, and their potential adverse effects on communities may be good reasons for limiting access and justify careful monitoring of certain substances. Overly strict regulations, fear of legal consequences, stigma associated with abuse and populations using illicit drugs, and lack of funding may hinder research on their considerable therapeutic potential. We review the surprisingly sparse literature and address the particular ethical concerns of undue inducement, informed consent, risk to participants, researchers and institutions, justice and liberty germane to the research with illicit and addictive substances. We debate the disparate research stakeholder perspectives and why they are likely to be infected with bias. We propose an empirical research agenda to provide a more evidentiary basis for ethical reasoning.