I'm currently reading the IEA's latest book, Prohibitions. Edited by John Meadowcroft, it tackles the whole range of prohibitions – from drugs and prostitution to the sale of body parts. It looks like an excellent (and timely) contribution to the public debate. You can get it here from the IEA website.
The introduction sketches out the general case against prohibition. The opening points are theoretical. Any restriction on what we are allowed to do with our own body "assigns partial ownership rights in citizens to the state", reducing individual liberty. Such prohibitions must therefore be considered very carefully, with "an assumption that government will not intervene, even if a good case for intervention can be made, other than as an absolute last resort." Without such an assumption, liberty can be gradually undermined by a series of well-intentioned and seemingly worthwhile interventions, "until it has completely disappeared."
Laws should instead be based on Mill's harm principle: the state should only prohibit things that directly harm others. Once that principle is breached, and autonomous individuals are prevented from freely choosing actions that only harm themselves, almost any intervention can be justified.
The chapter then outlines a series of practical arguments against prohibition. Firstly, prohibition places markets in criminal hands, imposing costs on the whole of society. Secondly, prohibition "increases the risk of already risky activities". Unlike legitimate corporations, drug producers have little incentive to ensure the safety of their product, and users must purchase drugs without adequate knowledge of their purity or contents. Similarly, prostitutes forced to work illegally are at much greater risk than those working in legal brothels.
Thirdly, prohibition criminalizes people who would not otherwise be criminals, by making illegal acts voluntarily undertaken by consenting adults. Fourthly, prohibitions divert police resources away from activities that actually harm third parties, imposing an opportunity cost on society and leading to higher taxes. Fifthly, prohibition increases public ignorance, making it harder for them to get the information required to assess the risks of a particular activity.
The upshot of all this is that "prohibition almost never works and is almost always counter-productive". I'm looking forward to reading the rest of the book.