Unfortunately, cannabis is a highly politicized drug, making it difficult to find unbiased sources.
I've scanned the scientific literature
briefly (20 min) and this is what I've found.
I have not evaluated these publications for scientific integrity but presumably since they're published they should be alright although any of you who are scientists will laugh and know this isn't true 100% of the time:
Neurological effects: memory, mental disorders, brain function, etc
[PDF] The chronic effects of cannabis on memory in humans: a review
bentham.org [PDF]N Solowij, R Battisti - Curr Drug Abuse Rev, 2008 - bentham.org
http://www.bentham.org/cdar/samples/cdar 1-1/Solowij.pdf
"[T]his review sought to examine the literature specific to memory function in cannabis users in the unintoxicated state with the aim of identifying the existence and nature of memory impairment in cannabis users and appraising potentially related mediators or moderators" and "Most recent studies have examined working memory and verbal episodic memory and cumulatively, the evidence suggests impaired encoding, storage, manipulation and retrieval mechanisms in long-term or heavy cannabis users. These impairments are not dissimilar to those associated with acute intoxication and have been related to the duration, frequency, dose and age of onset of cannabis use. We consider the impact of not only specific parameters of cannabis use in the manifestation of memory dysfunction, but also such factors as age, neurodevelopmental stage, IQ, gender, various vulnerabilities and other substance-use interactions, in the context of neural efficiency and compensatory mechanisms."
Regional Brain Abnormalities Associated With Long-term Heavy Cannabis Use
Murat Ycel, PhD, MAPS; Nadia Solowij, PhD; Colleen Respondek, BSc; Sarah Whittle, PhD; Alex Fornito, PhD; Christos Pantelis, MD, MRCPsych, FRANZCP; Dan I. Lubman, MB ChB, PhD, FRANZCP
Arch Gen Psychiatry. 2008;65(6):694-701.
http://archpsyc.ama-assn.org/cgi/content/abstract/65/6/694
"Conclusions: These results provide new evidence of exposure-related structural abnormalities in the hippocampus and amygdala in long-term heavy cannabis users and corroborate similar findings in the animal literature. These findings indicate that heavy daily cannabis use across protracted periods exerts harmful effects on brain tissue and mental health."
Long-term effects of frequent cannabis use on working memory and attention: an fMRI study
Psychopharmacology, 2006
Gerry Jager1 , Rene S. Kahn1, Wim Van Den Brink2, Jan M. Van Ree3 and Nick F. Ramsey1
http://www.springerlink.com/content/t7424196656k7161/
"No evidence was found for long-term deficits in working memory and selective attention in frequent cannabis users after 1 week of abstinence. Nonetheless, frequent cannabis use may affect brain function, as indicated by altered neurophysiological dynamics in the left superior parietal cortex during working memory processing."
Acute and Non-acute Effects of Cannabis on Brain Functioning and Neuropsychological Performance
Neuropsychology Review
Springer Science+Business Media, LLC 2007
10.1007/s11065-007-9036-8
Raul Gonzalez1
"Summary and Concluding Remarks
The past two decades have experienced remarkable breakthroughs in understanding the mechanisms by which cannabis produces its psychoactive effects. An endocannabinoid signaling system that includes cannabinoid receptors to which THC binds has been discovered and characterized in the human brain. This system has been implicated in regulation of various brain functions and shows great promise for therapeutic applications (Grant and Cahn 2005).
Scientific advances have also been made in understanding the neurobehavioral consequences of cannabis use through the use of neuroimaging. Current evidence clearly demonstrates changes in brain functioning among acutely intoxicated and abstinent cannabis users. The most consistent findings are of increased blood blow or hypermetabolism during acute intoxication and decreased blood flow or hypometabolism during recent abstinence, when participants are at rest. The duration of these changes after abstinence is less clear but may persist for weeks. Subtle differences in brain functioning have also been reported among abstinent cannabis users involved in various neurocognitive tasks; as expected, however, the specific brain regions affected vary according to task and across studies. These changes in brain functioning do not necessarily translate to functional impairments, since abstinent cannabis users in such studies often reveal no differences on task performance. Neither do differences in brain functioning translate to evidence for structural damage. Indeed, evidence for structural brain damage in humans as a result of cannabis use remains equivocal, and it may be that various yet unknown circumstances may affect whether cannabis will exert neurotoxic or neuroprotective effects.
Poorer performances on measures of neuropsychological functioning are also observed among cannabis users. Those who are acutely intoxicated show deficits in memory that appear specific to material presented during acute intoxication and usually only during free recall paradigms when recognition cues are not provided. Such deficits can be best characterized as retrieval-based, which would suggest disruptions of prefrontal brain circuits rather than frank damage to hippocampal structures. Consistent with this interpretation is evidence for intrusion and false positive errors during memory testing and functional neuroimaging studies that more often reveal disturbances in prefrontal structures than in the temporal lobes. It may be that disruptions in brain structures outside the mid-temporal lobes more likely mediate cannabis-related memory problems, which may be executive in nature. Poorer memory is also the most reliably detected deficit among abstinent cannabis users, which suggests that memory disturbances experienced during acute use persist after intoxication subsides. However, these deficits appear to be mild and transient.
At this time, the preponderance of evidence suggests circumscribed deficits of small magnitude that are most commonly seen with frequent and heavy use of cannabis. Individuals with such patterns of use may also be those most likely to be addicted to cannabis and meet criteria for a cannabis-related substance-use disorder. Although they represent only a small percentage of those that have ever tried cannabis, in the United States alone they numbered more than 4 million during 2005. Thus, cannabis use disorders are of significant concern for public health. Given the significant negative impact that drug addiction can have on an individuals life, it can be argued that one of the greatest neurobehavioral liabilities from cannabis use is its potential for addiction. These potential risks must be weighed in tandem with the developing therapeutic applications of cannabis and specific cannabinoids, since the risk for development of dependence and limited risk for neuropsychological deficits may not be sufficient to contraindicate use under many circumstances."
Cannabis use and risk of psychotic or affective mental health outcomes: a systematic review
The Lancet, Volume 370, Issue 9584, Pages 319-328
T. Moore, S. Zammit, A. Lingford-Hughes, T. Barnes, P. Jones, M. Burke, G. Lewis
"The evidence is consistent with the view that cannabis increases risk of psychotic outcomes independently of confounding and transient intoxication effects, although evidence for affective outcomes is less strong. The uncertainty about whether cannabis causes psychosis is unlikely to be resolved by further longitudinal studies such as those reviewed here. However, we conclude that there is now sufficient evidence to warn young people that using cannabis could increase their risk of developing a psychotic illness later in life."
Pulmonary and respiratory effects
Effects of Marijuana Smoking on Pulmonary Function and Respiratory Complications
A Systematic Review
Jeanette M. Tetrault, MD; Kristina Crothers, MD; Brent A. Moore, PhD; Reena Mehra, MD, MS; John Concato, MD, MS, MPH; David A. Fiellin, MD
Arch Intern Med. 2007;167(3):221-228.
http://archinte.highwire.org/cgi/content/abstract/167/3/221
"Conclusions: Short-term exposure to marijuana is associated with bronchodilation. Physiologic data were inconclusive regarding an association between long-term marijuana smoking and airflow obstruction measures. Long-term marijuana smoking is associated with increased respiratory symptoms suggestive of obstructive lung disease."
Effects of cannabis on pulmonary structure, function and symptoms
Thorax 2007;62:1058-1063 doi:10.1136/thx.2006.077081
http://thorax.bmj.com/content/62/12/1058.full
Other effects and summaries
Interestingly, these first two articles appear to contradict each other.
Long-term effects of exposure to cannabis
Leslie Iversen
Current Opinion in Pharmacology
Volume 5, Issue 1, February 2005, Pages 69-72
http://www.sciencedirect.com/scienc...d=582538&md5=5cd99271d53925bd4843f082baadfe42
"A review of the literature suggests that the majority of cannabis users, who use the drug occasionally rather than on a daily basis, will not suffer any lasting physical or mental harm. Conversely, as with other recreational drugs, there will be some who suffer adverse consequences from their use of cannabis. Some individuals who have psychotic thought tendencies might risk precipitating psychotic illness. Those who consume large doses of the drug on a regular basis are likely to have lower educational achievement and lower income, and may suffer physical damage to the airways. They also run a significant risk of becoming dependent upon continuing use of the drug. There is little evidence, however, that these adverse effects persist after drug use stops or that any direct cause and effect relationships are involved.
In contrast, cannabis might have beneficial effects in some medical indications. There is considerable literature obtained from animal studies to suggest that cannabis has analgesic effects [27]. Until recently, however, there has been a dearth of controlled clinical studies to validate such effects in patients. This has now changed, with the publication in the past two years of a number of double-blind placebo-controlled trials showing the effectiveness of cannabinoids in relieving chronic neuropathic pain 28. and 29. or pain associated with multiple sclerosis 30. and 31.. The largest of these trials involved 630 multiple sclerosis patients and showed significant pain relief after 15 weeks of treatment with either pure THC or cannabis extract [30]. It seems likely that medicinal cannabis will re-enter the Pharmacopoeia."
Long-term effects of cannabis
Anna Boycea, b and Paul McArdlea, b
Paediatrics and Child Health
Volume 18, Issue 1, January 2008, Pages 37-41
"Conclusion
There is reasonable evidence for a link between cannabis use and risk of overall adverse psychosocial functioning, lower educational attainment, development of psychosis, particularly in individuals vulnerable by virtue of family history, and possibly cognitive performance. Young people using cannabis on a regular basis should be warned of the possible consequences to their mental health in addition to the physical health risks of smoking cannabis. Health professionals working with adolescents should be prepared to ask young people about their drug use and be aware of local resources to support young people who wish to address their drug use.
Misuse of cannabis in adolescence may have long-term consequences on psychosocial functioning and mental health
Cannabis use is associated with later use of other illicit substances, although the mechanism for this is unclear
There is not enough evidence to enable prediction of outcome for individuals exposed to cannabis prenatally
Questions about substance misuse should form part of routine assessment by health professionals working with young people and referral to substance misuse services made as appropriate"
not for your health but in other people's minds.. 
and they cannot do that... and want to spread their unhappy waves all around hopefully they find someone to make them feel not alone in their view/opinion.. and they can get together negative as the "minus" itself.. and discussing other people's lives instead of living their own.. 
