Substance-related disorders involve substances that directly activate the brain's reward system. The activation of the reward system typically causes feelings of pleasure; the specific characteristics of the pleasurable feelings evoked vary widely depending on the drug. These drugs are divided into 10 different classes that have different, although not completely distinct, pharmacologic mechanisms. The classes of drugs include
Cannabis Marijuana (Cannabis) Marijuana is a euphoriant that can cause sedation or dysphoria in some users. Psychologic dependence can develop with chronic use, but very little physical dependence is clinically apparent... read more and synthetic cannabinoids Cannabinoids, Synthetic Synthetic cannabinoids are manufactured drugs that are tetrahydrocannabinol (THC) receptor agonists. They are typically applied to dried plant material and smoked. THC is the primary active... read more
Hallucinogens Hallucinogens Hallucinogens are a diverse group of drugs that can cause unpredictable, idiosyncratic reactions. Intoxication typically causes hallucinations, with altered perception, impaired judgment, ideas... read more (eg, LSD, phencyclidine Ketamine and Phencyclidine (PCP) Ketamine and phencyclidine are N-methyl-D-asparate receptor antagonists and dissociative anesthetics that can cause intoxication, sometimes with confusion or a catatonic state. Overdose can... read more , psilocybin, 3,4-methylenedioxymethamphetamine [MDMA])
Inhalants Volatile Solvents Inhalation of volatile industrial solvents and solvents from aerosol sprays can cause a state of intoxication. Chronic use can result in neuropathies and hepatotoxicity. Use of volatile solvents... read more (volatile hydrocarbons [eg, paint thinner, certain glues])
Stimulants (eg, amphetamines Amphetamines Amphetamines are sympathomimetic drugs with central nervous system stimulant and euphoriant properties whose toxic adverse effects include delirium, hypertension, seizures, and hyperthermia... read more , cocaine Cocaine Cocaine is a sympathomimetic drug with central nervous system stimulant and euphoriant properties. High doses can cause panic, schizophrenic-like symptoms, seizures, hyperthermia, hypertension... read more )
This classification is not based on whether a drug is legal (eg, alcohol, caffeine), illegal (eg, hallucinogens), or available by prescription (eg, morphine, lorazepam). Specific details regarding these drugs and their effects are discussed elsewhere in THE MANUAL.
The term "narcotic" is a legal and colloquial term. Originally, it referred to drugs that caused narcosis (insensibility or stupor), particularly opioid drugs (eg, opium, opium derivatives). However, the term is currently used so inconsistently (eg, the US government classifies the stimulant drug cocaine as a narcotic) that the term has little scientific or medical meaning.
Use and cessation of substances may cause psychologic, behavioral, and physiologic changes, ie, intoxication and withdrawal. Substance misuse can also result in psychiatric disorders (eg, depression Depressive Disorders Depressive disorders are characterized by sadness severe enough or persistent enough to interfere with function and often by decreased interest or pleasure in activities. Exact cause is unknown... read more , psychosis Introduction to Schizophrenia and Related Disorders Schizophrenia and related psychotic disorders— brief psychotic disorder, delusional disorder, schizoaffective disorder, schizophreniform disorder, and schizotypal personality disorder—are characterized... read more , anxiety Overview of Anxiety Disorders Everyone periodically experiences fear and anxiety. Fear is an emotional, physical, and behavioral response to an immediately recognizable external threat (eg, an intruder, a car spinning on... read more , or neurocognitive disorders).
Substance use disorders involve a pathologic pattern of behaviors in which patients continue to use a substance despite experiencing significant problems related to its use. There may also be physiologic manifestations, including changes in brain circuitry. The common terms "addiction," "abuse," and "dependence" are too loosely and variably defined to be very useful in systematic diagnosis; "substance use disorder" is more comprehensive and has fewer negative connotations.
Drugs in the 10 classes vary in how likely they are to cause a substance use disorder. The likelihood is termed addiction liability and depends upon a combination of factors including
Route of administration
Rate at which the drug crosses the blood-brain barrier and stimulates the reward pathway
Time to onset of effect
Ability to induce tolerance and/or withdrawal symptoms
In the US, the Comprehensive Drug Abuse Prevention and Control Act of 1970 and subsequent modifications require the pharmaceutical industry to maintain physical security of and strict record keeping for certain classes of drugs (controlled substances—see table ). Controlled substances are divided into 5 schedules (or classes) on the basis of their potential for abuse, accepted medical use, and accepted safety under medical supervision. The schedule classification determines how a substance must be controlled.
Schedule I: These substances have a high addiction liability, no accredited medical use, and a lack of accepted safety. They can be used only under government-approved research conditions.
Schedule II to IV: These drugs have progressively less addiction liability (going from schedule II to IV). They have an accredited medical use. Prescriptions for these drugs must bear the physician’s federal Drug Enforcement Administration (DEA) license number.
Schedule V: These substances have the least addiction liability. Some Schedule V drugs do not require a prescription.
State schedules may vary from federal schedules.