- Describe how substance abuse disorders are diagnosed
- Describe depressants and the impact of their use
- Describe stimulants and the impact of their use
- Describe opioids and the impact of their use
- Describe hallucinogens and the impact of their use
- Compare and contrast between depressants, stimulants, opioids, and hallucinogens
Substance Abuse
Psychologists and clinicians use a manual called the Diagnostic and Statistical Manual of Mental Disorders, or DSM, to diagnose and classify mental disorders. The manual is now on its fifth revised edition, called the DSM-5-TR (2022). Substance abuse disorders are are classified under “Substance-Related and Addictive Disorders.”
The diagnostic term is Substance Use Disorder (SUD), and it’s defined by 11 criteria considered over a 12-month period. Clinicians code severity by the number of criteria met: mild (2–3), moderate (4–5), severe (6+). The DSM-5-TR also provides specifiers such as in early remission, in sustained remission, in a controlled environment, and (for opioid use disorder) on maintenance therapy (e.g., methadone, buprenorphine, or naltrexone). Separate diagnoses exist for substance intoxication and withdrawal.
The Neurobiology of Addiction
Contemporary research views addiction as a chronic brain disorder characterized by specific neuroadaptations. These changes occur across a three-stage cycle: binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation. Each stage involves distinct brain regions and neurotransmitter systems, particularly dopamine, glutamate, and stress-response systems. Neuroimaging studies have mapped how these brain circuits change during addiction, showing that the process involves alterations in reward processing, executive function, and emotional regulation.
dependence, tolerance, and withdrawal
Physical dependence involves changes in normal brain and body chemistry leading to symptoms that are not primarily cognitive or emotional in nature. Users develop a tolerance to substances over time and will experience withdrawal upon cessation of the drug’s use.
In contrast, a person who has psychological dependence experiences a cognitive and emotional, rather than physical, need for the drug and may use the drug to relieve psychological distress.
Tolerance is linked to physiological dependence, and it occurs when a person requires more and more of the drug to achieve effects previously experienced at lower doses. Tolerance can cause the user to increase the amount of drug used to a dangerous level—even to the point of overdose and death.
Drug withdrawal includes a variety of negative symptoms experienced when drug use is discontinued. These symptoms usually are opposite of the effects of the drug. For example, withdrawal from sedative drugs often produces unpleasant arousal and agitation. Many individuals who are diagnosed with substance use disorders experience both tolerance and withdrawal symptoms.
Importantly, tolerance and withdrawal alone don’t equal an SUD: DSM guidance specifies that when these occur during appropriate medical treatment, they shouldn’t count toward the diagnosis. In short, physical dependence (e.g., needing coffee to avoid a headache) can exist without the impaired control and consequences that define an SUD.
According to 2024 data from SAMHSA’s National Survey on Drug Use and Health, approximately 48.4 million Americans (16.8% of those aged 12 or older) met criteria for a substance use disorder. Marijuana use disorder was most common (20.6 million), followed by opioid use disorder (4.8 million) and stimulant use disorder (4.3 million). Notably, only about 1 in 5 people needing treatment actually received it in 2024, highlighting a significant treatment gap.[1]
Drug Categories
The effects of all psychoactive drugs occur through their interactions with our endogenous neurotransmitter systems. As you have learned, drugs can act as agonists or antagonists of a given neurotransmitter system. An agonist facilitates the activity of a neurotransmitter system, and antagonists impede neurotransmitter activity.
The main categories of drugs are depressants, stimulants, and hallucinogens. You’ll learn more about these types of drugs in the coming pages.
| Class of Drug | Examples | Effects on the Body | Effects When Used | Psychologically Addicting? |
| Stimulants | Cocaine, amphetamines (including some ADHD medications such as Adderall), methamphetamines, MDMA (“Ecstasy” or “Molly”) (MDMA also acts as a mild hallucinogen) | Increased heart rate, blood pressure, body temperature | Increased alertness, mild euphoria, decreased appetite in low doses. High doses increase agitation, paranoia, can cause hallucinations. Some can cause heightened sensitivity to physical stimuli. High doses of MDMA can cause brain toxicity and death. | Yes |
| Sedative-Hypnotics (“Depressants”) | Alcohol, barbiturates (e.g., secobarbital, pentobarbital), Benzodiazepines (e.g., Xanax) | Decreased heart rate, blood pressure | Low doses increase relaxation, decrease inhibitions. High doses can induce sleep, cause motor disturbance, memory loss, decreased respiratory function, and death. | Yes |
| Opiates | Opium, Heroin, Fentanyl, Morphine, Oxycodone, Vicodin, methadone, and other prescription pain relievers | Decreased pain, pupil dilation, decreased gut motility, decreased respiratory function | Pain relief, euphoria, sleepiness. High doses can cause death due to respiratory depression. | Yes |
| Hallucinogens | Marijuana[2], LSD, Peyote, mescaline, DMT, psilocybin, dissociative anesthetics including ketamine and PCP | Increased heart rate and blood pressure that may dissipate over time | Mild to intense perceptual changes with high variability in effects based on strain, method of ingestion, and individual differences | Yes |
- Substance Abuse and Mental Health Services Administration. (2025). Key substance use and mental health indicators in the United States: Results from the 2024 National Survey on Drug Use and Health (HHS Publication No. PEP25-07-007, NSDUH Series H-60). Center for Behavioral Health Statistics and Quality, Substance Abuse and Mental Health Services Administration. https://www.samhsa.gov/data/data-we-collect/nsduh-national-surveydrug-use-and-health/national-release ↵
- Note that marijuana (cannabis) is most often included as a hallucinogen due to its psychoactive properties, but it can also be classified as a depressant or stimulant. ↵