Substance abuse, also known as drug abuse, is a patterned use of a substance (drug) in which the user consumes the substance in amounts or with methods which are harmful to themselves or others. The exact cause of substance abuse is impossible to know because there is not just one direct cause. However substance abuse and addiction is known to run in families. One theory suggests there is a genetic disposition which predisposes certain individuals toward substance abuse. Another suggests substance abuse is learned there for people subjected to substance abuse by those around them begin to copy the same behaviors. Substance abuse might start out as a bad habit but when and if addiction develops it manifests as a chronic debilitating disease.

The terms have a huge range of definitions related to taking a psychoactive drug or performance enhancing drug for a non-therapeutic or non-medical effect. All of these definitions imply a negative judgment of the drug use in question (compare with the term responsible drug use for alternative views). Some of the drugs most often associated with this term include alcohol, substituted amphetamines, barbiturates, benzodiazepines (particularly alprazolam, temazepam, diazepam and clonazepam), cocaine, methaqualone, and opioids. Use of these drugs may lead to criminal penalty in addition to possible physical, social, and psychological harm, both strongly depending on local jurisdiction.

There are many cases in which criminal or anti-social behavior occur when the person is under the influence of a drug. Long term personality changes in individuals may occur as well. Other definitions of drug abuse fall into four main categories: public health definitions, mass communication and vernacular usage, medical definitions, and political and criminal justice definitions. Substance abuse is prevalent with an estimated 120 million users of hard drugs such as cocaine, heroin, and other synthetic drugs.

Substance abuse is a form of substance-related disorder.