The use of recreational drugs of abuse continues to expand without limitations to national boundaries, social status, race, or education. Beyond the prevalence of illicit drug use and dependence, their contribution to the global burden of disease and death are large and troubling. All medical providers should be aware of the evolving drugs of abuse and their medical and social consequences. In addition to heroin and stimulants such as cocaine and methamphetamine, new designer stimulants called "bath salts" and cannabinoids called "spice," along with the abuse of prescription drugs and volatile substances, are now widely recognized problems in many societies. The wide variety and continuingly expanding clinical manifestations of toxicity of recreational drugs of abuse is not widely appreciated by clinicians. This edition attempts to summarize six major classes of drugs of abuse and their clinical effects with special emphasis on their immunological and respiratory effects.