As shown by the growing numbers of users attending the public drug addiction services, drug abuse is a phenomenon that is constantly spreading. It is important that dentists are aware of the oral problems linked to drug abuse. This study examines the general effects and oral implications of the illegal substances used by the majority of drug addicts. The main dental complications of cannabinoids are the increased incidence of squamous cell carcinomas of the oral cavity, the presence of xerostomia and severe gingivitis. Depending on how it is taken, cocaine may cause ischemic necrosis of the palate, inflammation, ulceration and gingival retraction, as well as an increased incidence of bruxism. Hallucinogens have few direct oral effects, but among these it is worth recalling xerostomia, increased bruxism and oral problems linked to malnutrition caused by ecstasy. Turning to the opioids, heroin is the drug primarily used by the majority of drug addicts. Its oral effects mainly take the form of dental decay, showing a particular form and extent linked either directly or indirectly to heroin use. This results in "typical" or "atypical" caries pathologies directly linked to the effects of heroin. Given the extent of this phenomenon, it is important that dentists are aware of the problems linked to drug abuse that they may have to treat.