Publisher Summary The prevalence of recreational drug use (especially in the young) and the potency of the available products have markedly increased in recent years. This chapter discusses several case studies explaining the adverse effects of methylenedioxymethamphetamine (MDMA) on the nervous system and the electrolyte balance. The immunologic effects of MDMA are also reviewed. The chapter also discusses the adverse effects of cannabinoids on the cardiovascular system and the nervous system. The adverse effects of cocaine on the cardiovascular system, the nervous system, gastrointestinal system, the urinary tract, and sexual function are also reviewed in the chapter. The immunologic effects, carcinogenicity, and fetotoxicity of cocaine usage are described. The effects of prenatal maternal cocaine use on neurobehavior of two-week-old infants are discussed. The adverse effects of opiates on the nervous system are also presented.