Amotivational syndrome is a psychological condition associated with diminished inspiration to participate in social situations and activities, with episodes of apathy caused by an external event, situation, substance (or lack of), relationship (or lack of), or other cause.
While some have claimed that chronic use of cannabis causes amotivational syndrome in some users, empirical studies suggest that there is no such thing as "amotivational syndrome", per se. From a World Health Organization 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 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. 
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.
- Barnwell, S. S., Earleywine, M. & Wilcox, R. 2006. "Cannabis, motivation, and life satisfaction in an internet sample," Substance Abuse Treatment, Prevention, and Policy, 1(2). Via NCBI PubMed.
- Marin RS. Apathy and related disorders of diminished motivation. American Psychiatric Press Review of Psychiatry. 1996;15:205–242.