Amotivational syndrome is a psychological condition associated with diminished inspiration to participate in social situations and activities, with lapses in apathy

Apathy

Apathy is a state of indifference, or the suppression of emotions such as concern, excitement, motivation and passion. An apathetic individual has an absence of interest in or concern about emotional, social, spiritual, philosophical or physical life.They may lack a sense of purpose or meaning in...

 caused by an external event, situation, substance (or lack of), relationship, or other cause.

While some have claimed that chronic use of cannabis

Cannabis (drug)

Cannabis, also known as marijuana among many other names, refers to any number of preparations of the Cannabis plant intended for use as a psychoactive drug or for medicinal purposes. The English term marijuana comes from the Mexican Spanish word marihuana...

 causes amotivational syndrome in some users, empirical

Empirical

The word empirical denotes information gained by means of observation or experimentation. Empirical data are data produced by an experiment or observation....

 studies suggest that there is no such thing as "amotivational syndrome", per se, but that chronic cannabis intoxication can lead to apathy and amotivation.
From a World Health Organization

World Health Organization

The World Health Organization is a specialized agency of the United Nations that acts as a coordinating authority on international public health. Established on 7 April 1948, with headquarters in Geneva, Switzerland, the agency inherited the mandate and resources of its predecessor, the Health...

 report:

The evidence for an "amotivational syndrome" among adults consists largely of case histories and observational reports (e.g. Kolansky and Moore, 1971; Millman and Sbriglio, 1986). The small number of controlled field and laboratory studies have not found compelling evidence

Evidence

Evidence in its broadest sense includes everything that is used to determine or demonstrate the truth of an assertion. Giving or procuring evidence is the process of using those things that are either presumed to be true, or were themselves proven via evidence, to demonstrate an assertion's truth...

 for such a syndrome (Dornbush, 1974; Negrete, 1983; Hollister, 1986)... (I)t is doubtful that cannabis use produces a well defined amotivational syndrome. It may be more parsimonious to regard the symptoms of impaired motivation as symptoms of chronic cannabis intoxication rather than inventing a new psychiatric syndrome. http://hyperreal.info/~emes/WHO/who-probable.html

A study done by researchers Barnwell, Earleywine and Wilcox on a sample of undergraduates also suggests that cannabis use does not cause an amotivational syndrome. The e-mail survey showed no significant difference in motivation (as measured on the Apathy Evaluation Scale) between cannabis users and cannabis abstainers.