Substance-related disorder
Substance-related disorders, including both substance dependence and substance abuse, can lead to large societal problems. It is found to be greatest in individuals ages 18–25, with a higher likelihood occurring in men compared to women, and urban residents compared to rural residents. On average, general medical facilities hold 20% of patients with substance-related disorders, possibly leading to psychiatric disorders later on. Over 50% of individuals with substance-related disorders will often have a "dual diagnosis," where they are diagnosed with the substance abuse, as well as a psychiatric diagnosis, the most common being major depression, personality disorder, anxiety disorders, and dysthymia.

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 drugs used are often associated with levels of intoxication that alter judgment, perception, attention and physical control, not related with medical  effects.  It is often thought that the main abused substances are illegal drugs and alcohol; however it is becoming more common that prescription drugs and tobacco are a prevalent problem.

Signs and symptoms

Common symptoms include:
	Sudden changes in behaviour – may engage in secretive or suspicious behaviour 
	Mood changes – anger towards others, paranoia and little care shown about themselves or their future
	Problems with work or school – lack of attendance 
	Changes in eating and sleeping habits 
	Changes in friendship groups and poor family relationships 
	A sudden unexplained change in financial needs – leading to borrowing/stealing money

There are many more symptoms such as physical and psychological changes, though this is often dependent on which drug is being abused. It is, however, common that abusers will experience unpleasant withdrawal symptoms if the drug is taken away from them.

It is also reported that others have strong cravings even after they have not used the drug for a long period of time. This is called being "clean". To determine how the brain triggers these cravings, multiple tests have been done on mice.  It is also now thought that these cravings can be explained by substance-related disorders as a subcategory of personality disorders as classified by the DSM-5.

Classification and terminology

Substance-related disorders were originally subcategorized into "substance use disorders" (SUD) and "substance-induced disorders" (SID). Though DSM-IV makes a firm distinction between the two, SIDs often occur in the context of SUDs.

Substance-induced disorders

Substance-induced disorders include medical conditions that can be directly attributed to the use of a substance. These conditions include intoxication, withdrawal, substance-induced delirium, substance-induced psychosis, and substance-induced mood disorders.

Substance use disorders

Substance use disorders include substance abuse and substance dependence. In DSM-IV, the conditions are formally diagnosed as one or the other, but it has been proposed that DSM-V combine the two into a single condition called "Substance-use disorder".

Changes in Classification

The more recently published DSM-5 combined substance abuse and substance dependence into a single continuum; this is simply known as substance use disorder and requires more presenting symptoms before a diagnosis is made.  It also considers each different substance as its own separate disorder, based upon the same basic criteria.  It also distinguishes the difference between dependence and addictions as two separate disorders, not to be confused.

Complications

There are many potential complications that can arise due to substance abuse such as severe physiological damage, psychological changes and social changes that are often not desirable.

Physiological damage is often the most obvious, observed as an abnormal condition affecting the body of an organism: For instance, there are several known alcohol-induced diseases (e.g. alcoholic hepatitis, alcoholic liver disease, alcoholic cardiomyopathy.)  Substance abuse is also often associated with premature ageing, fertility complications, brain damage and a higher risk of infectious diseases due to a weakened immune system.

Long term abuse has been linked to personality changes such as depression, paranoia, anxiety which can be related to psychological disorders. It is often reported that substance abuse coincides with personality disorders, such as borderline personality disorder. It has also now been linked to severe brain damage leading to an inability to control behaviours, which could explain why many people who abuse substances go on to develop addictions.

Substance abuse is often regarded as negative in society and therefore those who engage in such behaviours can often be subject to social discrimination. The use of many drugs can lead to criminal convictions, whether the drug itself is illegal or abusers use unlawful methods to fund their substances. It is also more likely that someone will partake in criminal or anti-social behaviour when they are under the influence of a drug.