Merck Manual

Please confirm that you are a health care professional

honeypot link

Overview of Substance Use


Mashal Khan

, MD, NewYork-Presbyterian Hospital

Reviewed/Revised Oct 2022 | Modified Dec 2022
Topic Resources

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

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.

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

Scheduled drugs

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.


Drugs Mentioned In This Article

Drug Name Select Trade
Cafcit, NoDoz, Stay Awake, Vivarin
ABSTRAL, Actiq, Duragesic, Fentora, IONSYS, Lazanda, Onsolis, Sublimaze, SUBSYS
ARYMO ER, Astramorph PF, Avinza, DepoDur, Duramorph PF, Infumorph, Kadian, MITIGO, MORPHABOND, MS Contin, MSIR, Opium Tincture, Oramorph SR, RMS, Roxanol, Roxanol-T
Dazidox , Endocodone , ETH-Oxydose, Oxaydo, OXECTA, OxyContin, Oxydose , OxyFast, OxyIR, Percolone, Roxicodone, Roxybond, XTAMPZA
Ativan, Loreev XR
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
quiz link

Test your knowledge

Take a Quiz!