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 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.
Other research done by Imperial College London  has found that long-term users of cannabis tend to produce less dopamine, a chemical produced by the brain which plays an important role in executive functions, motor control, motivation, arousal, reinforcement, and reward. The research also suggests that users of cannabis which started at a younger age may produce even less dopamine than those that began using later in their life. The cannabis users chosen for the study had all experienced psychotic-like episodes while using the drug and were selected as such because previous research hypothesizes that positive symptoms of psychosis are linked to excess dopamine production. However, the researchers found that the opposite was the case in these long-term cannabis users. Because dopamine plays a large role in motivation, the research suggests that long-term cannabis use may cause amotivational syndrome.
- 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.
- Preston, John; John, O'Neal; Mary, Talaga (2013). Handbook of Clinical Psychopharmacology for Therapists (7th ed.). New Harbinger Publications. p. 193. ISBN 1608826643.