The drug induced psychosis seen when Cannabis is the main substance being abused is distinct phenomenologically from other psychoses.  It is unusual for such a psychosis to occur without other drugs being involved to some extent and so it is difficult to tease out the difference.
However it is misleading and dangerous, to our youth in particular, to label Cannabis as “soft”.  In fact the serious adverse effects of Cannabis have been known for some time now and Hall and Solowij in the British Journal of Psychiatry sounded warnings in 1997 about such issues as dependence on Cannabis, adolescent developmental problems, permanent cognitive impairment as well as involvement in and the development of psychosis. [1]
There have been suggestions that such people may be the ones who have started Cannabis in their teens and caused disturbance to neural connectivity.  However, it seems Cannabis can precipitate or exacerbate a schizophrenic tendency in a characteristic manner.[2]

Chronic Symptoms of Cannabis Psychosis
Patients are left with the well-recognised and permanent symptoms of memory loss, apathy, loss of motivation and, paranoid ideation.  These symptoms known as “the Amotivational Syndrome” in the past are usually permanent.[3]  If Cannabis using resumes then the acute symptoms redevelop.  The chronic state can also be arrived at without a preceding psychotic episode.  After Cannabis started to be widely used about 20 years ago, for permanent damage to occur it was felt by some that Cannabis had to be heavily used over at least three years [4].  However, there is accumulating evidence that smaller amount will do damage also and in animals “deficits on tasks dependent on frontal lobe function have been reported in cannabis users”[5].  It is very difficult to conduct research in this area, as it is not acceptable to harm humans by doing trials with damaging substances such as Cannabis.
However there is accumulating evidence of the psychological consequences of using Cannabis [6].  It is logical that to get the permanent “Amotivational Syndrome” small amounts to damage have to accumulate incrementally.  All this is in addition to the recognised danger of a recurrence of a pre-existing illness, such as Schizophrenia or Manic-depressive disorder.  There are suggestions that Cannabis “caused schizophrenia in young people and (or) enhanced the symptoms, especially in young people poorly able to cope with stress or in whom the antipsychotic therapy was unsuccessful”.[7]  Caspari found “patients with previous cannabis abuse had significantly more rehospitalizations, tended to worse psychosocial functioning, and scored significantly higher on the psychopathological syndromes “thought disturbance” (BPRS) and “hostility” (AMDP).  These results confirm the major impact of cannabis abuse on the long-term outcome of schizophrenic patients”.[8]
References:
[1] Hall W, Solowij N, “Long-term Cannabis use and Mental Health” 1997 British Journal of Psychiatry, August, 171:107-8
[2] HaIl A, Degenhardt, “Cannabis and Psychosis” Australian National Drug and Alcohol Research Centre, Presented at The Inaugural International Cannabis and Psychosis Conf. 1999, Melbourne 16-17 February 1999
[3] Schwartz RH “Marijuana: an overview”. Pediatr Clin North Am 1987 Apr;34(2):305-17.
[4] Boettcher B, Medical Journal of Australia 11/25 December 1982 “Marijuana and Apathy”
[5] Jentsch J D, Verrico C D, Le D, Roth RH, “Repeated exposure to dleta9-tetragydrocannabinol reduces prefrontal cortal dopamine metabolism in the rat” ,Neurosci Left (1998) May 1;246(3):169-72
[6] Hall W, Solowji N, Lemon J, The health and psychological consequences Monograph Series no 25. Canberra:Australia Government Publishing Service, 1994 of Cannabis use. Nat. Drug Strategy
[7] van Amsterdam JG, et al “Cognitive and psychotic effects after cessation of chronic cannabis use” Ned Tijdschr Geneeskd 1998 Mar 7: 142(10):504-8
[8] Caspari D, “Cannabis and Schizophrenia: Results of a follow-up  Study” Eur.Arch Psychiatry Clinical Neuroscience 1999: 249 (1):45-9