Many clinicians are confronted by the use of illicit drugs on a daily basis. The unsanctioned use of opioids, psychostimulants, benzodiazepines, alcohol and nicotine is a major cause of morbidity and mortality. Multiple factors have inhibited the scientific study of these agents including prohibition, public denial and lack of commercial interests. In dealing with problems related to these drugs, clinicians need a scientific understanding of their pharmacology, quantifiable effects and potential adverse effects. Illicit drug users select drugs with particular pharmacokinetic parameters and pharmacodynamic properties. Generally, rapid absorption, rapid entry into the central nervous system, high bioavailability, short half-life, small volume of distribution and high free drug clearance are pharmacokinetic characteristics which predict a high potential for harmful use because these factors increase positive reinforcement. Drug users adapt the method and route of drug administration to optimise the delivery of the drug to the brain while attempting to maximise the bioavailability of the drug. Inhalation and smoking are the routes of administration which allow the most rapid delivery of drug to the brain, while intravenous injection maximises the bioavailability of an administered drug. Each route of administration results in attendant complications related to mucosal damage, carcinogenesis and risk of infection. Negative reinforcement or withdrawal is a major drive to recurrent use. Many illicit drugs have pharmacological features that promote dependence, including long half-life, low free drug clearance and sufficient drug exposure to allow development of tolerance. The preventive or reductive pharmacotherapeutics of illicit drug use makes use of several subsets of agents: those which act on the same receptor or system as the illicit drug (such as methadone), those which produce an adverse reaction on consumption of the illicit drug (such as disulfiram) and those which symptomatically attenuate illicit drug withdrawal symptoms (such as clonidine). Many new agents are being trialled as potential preventive or reductive agents. It is important to consider pharmacotherapy as only one potential part of the treatment of illicit drug users. The complications of illicit drug use present many therapeutic challenges. As with all patients consuming multiple drugs, illicit drug users are prone to developing drug interactions. The most common interactions seen in practice are pharmacodynamic in nature, most often due to the additive effects of different drugs on the central nervous system. However, alcohol, cocaine, disulfiram, methadone and tricyclic antidepressants may be involved in important pharmacokinetic interactions. Of these the effect of long term alcohol consumption in increasing the hepatotoxicity of paracetamol and of cytochrome P450 3A microsomal enzyme stimulating drugs in diminishing the efficacy of methadone are the most commonly encountered.