Does addiction to heroin undermine the voluntariness of heroin addicts' consent to take part in research which involves giving them free and legal heroin? This question has been raised in connection with research into the effectiveness of heroin prescription as a way of treating dependent heroin users. Participants in such research are required to give their informed consent to take part. Louis C. Charland has argued that we should not presume that heroin addicts are competent to do this since heroin addiction by nature involves a loss of ability to resist the desire for heroin. In this article, I argue that Charland is right that we should not presume that heroin addicts are competent to consent, but not for the reason he thinks. In fact, as Charland's critics correctly point out, there is plenty of evidence showing that heroin addicts can resist their desire for heroin. These critics are wrong, however, to conclude from this that we should presume that heroin addicts are competent to give their voluntary consent. There are, I shall argue, other conditions associated with heroin addiction that might constrain heroin addicts' choice in ways likely to undermine the voluntariness of their consent. In order to see this, we need to move beyond the focus on the addicts' desires for heroin and instead consider the wider social and psychological circumstances of heroin addiction, as well as the effects these circumstances may have on the addicts' beliefs about the nature of their options.