Illicit drug use is a prevalent and pervasive issue in the United States and, as a result, substance use disorders (SUDs) incur major costs to individuals, families, and society at large. In 2008 approximately 8.9% (22.2 million) of Americans age 12 or older were classified with substance dependence or abuse in the past year, and these estimates have remained constant over the past 7 years (Substance Abuse and Mental Health Services Administration [SAMHSA], 2009a). The financial burden on society is estimated to be $484 billion annually in substance use–related treatment and prevention, health care expenditures, lost wages, reduced job production, accidents, and crime. In addition to the substantial economic cost of substance use, SUDs are associated with engagement in multiple health-compromising and risk-taking behaviors (e.g., condom nonuse, multiple partners, impulsive spending, driving while intoxicated) that contribute to the significant public health costs associated with SUDs (Office of National Drug Control Policy [ONDCP], 2004).

SUDs can result from abuse of a single drug or multiple substances. Commonly, drugs of abuse are grouped based on categories that vary in their physiological and behavioral effects. In this section we discuss the different drug categories, including street and slang names, physiological and psychological effects of each category, and the withdrawal symptoms that occur as tolerance and dependence develop. Interested readers are referred to Julien (2004) for an extensive review.

Cannabinoids, such as marijuana and hashish (street names include dope, pot, weed, grass, hash), produce mild euphoria, sedation, enhanced sensory perception, increased appetite and pulse, psychomotor impairment, and confusion. Tolerance can occur with habitual use, and discontinuation of use can result in uncomfortable withdrawal effects including anxiety, depression, irritability, and insomnia. Alternatively, hallucinogens, which include LSD (acid, blotter), mescaline (buttons, peyote, mesc), and psilocybin (magic mushrooms, shrooms), create an altered state of consciousness, detachment from self and environment, and dissociative symptoms. Hallucinogens are not physically addictive; however, LSD and mescaline produce negative physiological reactions such as increased body temperature, blood pressure, and heart rate. Psychologically, hallucinogen use can result in persistent mental disorders characterized by panic attacks and psychosis.

Both cannabinoids and hallucinogens are illegal street drugs, but central nervous system (CNS) depressants, including alcohol, benzodiazepines (e.g., alprazolam [Xanax], diazepam [Valium], lorazepam [Ativan], clonazepam [Klonopin]; benzos, xannies), and barbiturates (Seconal, Amytal, Phenobarbital; reds, yellows, yellow jackets, barbs) are legal substances that can be obtained with a doctor’s prescription. With low or moderate doses, they produce euphoria and disinhibition as well as decreased respiration, pulse, and blood pressure. At higher doses, confusion, impaired judgment, coordination, and memory loss occur. CNS depressants are particularly dangerous when combined because this increases the risk of respiratory depression and arrest. With long-term use, tolerance occurs and discontinuation of use can cause symptoms ranging from anxiety, insomnia, nausea, and muscle tension to more severe symptoms such as seizures, hallucinations, and psychosis.

Opioids are often prescribed for pain relief (e.g., codeine, fentanyl, oxycodone), but these drugs are also commonly abused. Other drugs in this category include opium (big O, tar) and heroin (dope, H, junk, smack). The effects of intoxication include euphoria, sedation, drowsiness, confusion, nausea, constipation, and respiratory depression. High doses of opioids can lead to coma and death. With prolonged use, tolerance occurs and, in the absence of the drug, users experience craving, sweating, fever, diarrhea, vomiting, and pain.

Unlike CNS depressants and opiates, stimulants increase respiration, heart rate, and blood pressure, and decrease appetite. Stimulant intoxication produces euphoria, mental alertness, and increased energy. Impulsive behavior, aggressiveness, anxiety, and irritability are also common features experienced by users. Additionally, prolonged use or high doses of stimulants can result in stimulant-induced psychosis. Due to the stress placed on the cardiovascular system, stimulant use can cause cardiac arrest, stroke, and death. Stimulants include cocaine (crack, coke, blow, yayo), methamphetamine (meth, speed, crystal, crank), MDMA (ecstasy, E, X, Adam), amphetamines (Adderall, Dexedrine; speed), and methylphenidate (Ritalin, vitamin R). Nicotine, found in cigarettes, cigars, and smokeless tobacco (snuff, dip, spit) is also a stimulant. Tolerance occurs quickly and can often lead to dependence. Withdrawal symptoms include anxiety, anhedonia, irritability, insomnia, and depression.

Dissociative anesthetics, such as ketamine (Special K, vitamin K) and PCP (angel dust, hog, love boat, sherms), create a dream-like state, euphoria, numbness, increased heart rate and blood pressure, and impaired memory and motor function. At high doses, ketamine can cause delirium, respiratory depression, and arrest. PCP use can result in panic, aggression, depression, and violence. Users of dissociative anesthetics quickly experience increasing tolerance, and a permanent tolerance may develop after several months of use. Although tolerance occurs, these drugs do not appear to have withdrawal symptoms or dependence.

Inhalants include a range of solvents (glues, paint thinners, gasoline), gases (propane, butane, aerosol propellants, nitrous oxide; laughing gas, whippets), nitrates (isobutyl, isoamyl; poppers, snappers), and aerosols (hair spray, spray paint). Inhalants are “huffed” through the nose and mouth and enter the lungs and subsequently the bloodstream rather quickly. The effects of intoxication include loss of motor skills and inhibition, slurred speech, headache, nausea, wheezing, and loss of consciousness. Extended use can lead to muscle weakness, memory impairment, depression, damage to the nervous and cardiovascular systems, and sudden death. There is little known about tolerance, withdrawal, and dependence in relation to inhalants.

In summary, once an SUD is present, the withdrawal experience from different substances varies by the class of drug. In general, common psychological features of withdrawal involve symptoms of depression and anxiety. Although the physical symptoms of withdrawal can be difficult to tolerate, they are often short lived.

The Diagnostic and Statistical Manual (DSM-5; American Psychiatric Association [APA], 2013) provides a comprehensive classification system for the assessment and subsequent diagnosis of a substance use disorder across 10 drug classes including alcohol, cannabis, phencyclidine, other hallucinogens, inhalants, opioids, sedatives, stimulants, tobacco, and other/unknown. The DSM-5 diagnostic criteria for a substance use disorder require evidence of continued use in spite of significant substance-related problems. A key characteristic of a substance use disorder is a change in the user’s neurological circuitry, a side effect that persists even in times of post-use sobriety. This altered circuitry is believed to manifest itself in the user’s behavior and can therefore be linked to relapse.

The DSM-5 diagnostic criteria for substance use disorders specify a maladaptive pattern of behaviors related to substance use. These behaviors fall into four categories: impaired control, social impairment, risky use, and pharmacological criteria. Impaired control is often characterized by unsuccessful efforts to follow through with intended plans to cut back on use, taking more than planned, or use for a greater duration than initially planned. Impaired control can involve craving, a perceived need to engage in substance use, often arising in the context of prior use (e.g., the presence of people with whom one had used; being in a location where one had previously used). Craving is operationalized as the extent to which the desire to use consumes the patient’s thoughts. Social impairment pertains to an inability to carry out the needs of daily life in all facets of one’s environment, whether it involves failing to fulfill obligations or opting to use rather than engaging in social activities. Further, in spite of the negative impact that using may have, social impairment is characterized by persistent substance use in spite of the negative social impact. Risky use involves the continued use of a substance that is known to present a danger to the user. This risk might entail using in a dangerous situation or using with a known vulnerability to a physical or psychological issue that would likely be made worse by further use. Pharmacological criteria focuses on tolerance and withdrawal. Tolerance is a state that develops wherein the user needs progressively larger doses of the drug in order to feel the desired effect. Withdrawal is a physical response to specific substances that can occur after extended, consistent use. The diagnostic criteria for substance use disorders specify that at any given time the presence of two to three symptoms suggest mild substance abuse disorder with moderate (four to five symptoms) and severe disorders (six or more symptoms) requiring greater symptomology.

Another system, the International Classification of Diseases, 10th revision (ICD-10; World Health Organization [WHO], 1994), is considered the international standard diagnostic classification system for all general epidemiological and many health management purposes, including the analysis of the general health situation of population groups and monitoring of the incidence and prevalence of diseases in relation to social, biological, and interpersonal variables. Although there is overlap in the way that the DSM and ICD view substance use disorders, the two systems have had different paradigms for maladaptive substance use that do not align well. Most notably, the DSM-5 characterizes substance use by negative social consequences of recurrent or continued use, whereas the ICD-10 includes a category for harmful use (a nonresidual category), which requires demonstrable physical or psychological harm. The emphasis on physical or psychological harm (rather than legal and social) in the ICD-10 stems from a need for developing criteria that can be applied uniformly across different countries and cultures.

Substance use disorders co-occur with many clinical disorders, ranging from depression and anxiety disorders to personality disorders. National epidemiological data suggest that among individuals with any SUD, the prevalence for any mood or anxiety disorder is 40.9% and 29.9% respectively (Conway, Compton, Stinson, & Grant, 2006). Mood and anxiety disorders are the most common comorbidities, followed by antisocial personality disorder (Jane-Llopis & Matysina, 2006) and schizophrenia-spectrum disorders (Kushner, Abrams, & Borchardt, 2000). Among personality disorders, borderline and antisocial personality disorders have the highest rates of co-occurrence with SUDs, with estimates ranging from 5% to 32% and 14% to 69%, respectively (e.g., Goldstein et al., 2007; Trull, Sher, Minks-Brown, Durbin, & Burr, 2000). In fact, estimates from the Epidemiologic Catchment Area (ECA) Survey suggest that SUDs are more strongly associated with antisocial personality disorder than with any other mood, anxiety, or thought disorder (Regier et al., 1993). Additionally, prevalence of disorders differs by substance. For example, among individuals with lifetime opioid abuse or dependence, nearly 50% met the criteria for a personality disorder (Grella, Karno, Warda, Niv, & Moore, 2009). These statistics are particularly alarming given that individuals with co-occurring disorders generally have worse treatment outcomes (e.g., noncompliance and relapse), higher rates of suicidal ideation, distorted perception and cognition, social exclusion, aggression, and homelessness (Horsfall, Cleary, Hunt, & Walter, 2009). The causal direction of co-occurring conditions is mixed, with some evidence of mental disorders predicting the onset of SUDs, while other studies have found that SUDs predict later mental illness (Kessler, 2004; National Institute on Drug Abuse [NIDA], 2009).

Approximately 20.1 million (8%) Americans age 12 or older report illicit drug use in the past month, of which 57.3% report using only marijuana and 42.7% report illicit drug use other than marijuana (SAMHSA, 2009b). Specific populations are more vulnerable to initiation and continued illicit drug use. In particular, youth and young adults are a special population of interest, because early use increases the likelihood of future substance use problems. Data from the Monitoring the Future Study indicates that although use of several drugs (including inhalants, ecstasy, and amphetamines) has declined over the past decade, within the past 2 years marijuana use has increased. Approximately 12% of 8th graders, 27% of 10th graders, and 33% of 12th graders reported marijuana use in the past year. Respectively, about 15%, 29%, and 37% reported using any illicit drug in the past year (Johnston, O’Malley, Bachman, & Schulenberg, 2010).

In 2007, there were more than 1.8 million admissions to treatment, with the majority of these admissions being for alcohol only or alcohol and a secondary substance (SAMHSA, 2009b). Among the different treatment settings available, most admissions were in ambulatory care (62.3%), which is made up of outpatient (49.4%), intensive outpatient (10.6%), and detoxification (2.2%). Additionally, 18% of admissions entered residential treatment, with the majority in short-term treatment (less than 31 days), and 19.7% of admissions were for short detoxification (24-hour service) (SAMHSA, 2009b).