Below are a sample of 30 studies and statements, of over 20,000, on the harms of marijuana. More found here.

1. Marijuana use creates neurocognitive impairments and cannabis intoxication in both frequent and infrequent users. 
-- Journal of Scientific Reports, May 2016 (Cannabis and Tolerance: Acute Drug Impairment as a Function of Cannabis Use History).

2. Prevalence of cannabis use is expected to increase if cannabis is legal to use and legally available.
-- International Journal of Drug Policy, May 2016 (Correlates of Intentions to Use Cannabis among US High School Seniors in the Case of Cannabis Legalization)

3. Regular exposure to cannabis is associated with neuroanatomic alterations in several brain regions.
-- Journal of Biological Psychiatry, April 2016 (The Role of Cannabinoids in Neuroanatomic Alterations in Cannabis Users)

4. Marijuana is addicting, has adverse effects upon the adolescent brain, is a risk for both cardio-respiratory disease and testicular cancer, and is associated with both psychiatric illness and negative social outcomes.
-- Statement of the American College of Pediatricians, April 2016 (Marijuana Use: Detrimental to Youth).

5. Marijuana use has significant neuropharmacologic, cognitive, behavioral, and somatic consequences.
-- Statement of the American Academy of Pediatrics, March 2015 (The Impact of Marijuana Policies on Youth: Clinical, Research, and Legal Update)

6. Marijuana use is associated increased incidence and worsened course of psychotic, mood, anxiety, and substance use disorders across the lifespan…and marijuana’s deleterious effects on adolescent brain development, cognition, and social functioning may have immediate and long-term implications.
-- Statement of the American Academy of Child & Adolescent Psychiatry, 2014 (AACAP Marijuana Legalization Policy Statement)

7. Marijuana use may cause impairment in memory, concentration, and executive functioning…and may lead to permanent nervous system toxicity.
-- Statement of the American Academy of Neurology (Position Statement: Use of Medical Marijuana for Neurologic Disorders)

8. There is a strong association of cannabis use with the onset of psychiatric disorders. Adolescents are particularly vulnerable to harm, given the effects of cannabis on neurological development.
-- Statement of the American Psychiatric Association, December 2013 (Position Statement on Marijuana as Medicine)
 

9. Both marijuana-related hospitalizations and ED visits have increased substantially in recent years.                                                 -- Newsletter of the American College of Physicians, January 2016 (Public Health Researchers Look at Rise in Marijuana-related Hospitalizations)

10. Cannabis dependence is not associated with fewer harmful economic and social problems than alcohol dependence.
-- Journal of Clinical Psychological Science, June 2016 (Persistent Cannabis Dependence and Alcohol Dependence Represent Risks for Midlife Economic and Social Problems: A Longitudinal Cohort Study)

11. Repeated exposure to cannabis during adolescence may have detrimental effects on brain resting functional connectivity, intelligence, and cognitive function.
-- Journal of the Cerebral Cortex, February 2016 (Adverse Effects of Cannabis on Adolescent Brain Development: A Longitudinal Study)

12. Negative health effects of marijuana use can include addiction, abnormal brain development, psychosis, and other negative outcomes.
-- New England Journal of Medicine, June 2014 (Adverse Health Effects of Marijuana Use)

13. One in six infants and toddlers admitted to a Colorado hospital with coughing, wheezing and other symptoms of bronchiolitis tested positive for marijuana exposure.
-- American Academy of Pediatrics, April 2016 (One in Six Children Hospitalized for Lung Inflammation Positive for Marijuana Exposure)

14. Study respondents who were high had higher odds driving while intoxicated (on either marijuana or alcohol).
-- Journal of Health Education Research, April 2016 (Association Between Self-reports of Being High and Perceptions About the Safety of Drugged and Drunk Driving)

15. Cannabis use during adolescence increases the risk of developing a psychiatric disorder in adulthood, including anxiety, depression, and schizophrenia.
-- Frontiers in Neuroscience, November 2014 (Long-term Consequences of Adolescent Cannabinoid Exposure in Adult Psychopathology)

16. Childhood exposure to marijuana increases in marijuana friendly states and can lead to coma, decreased breathing, or seizures.
-- Journal of Clinical Pediatrics, June 2015 (Marijuana Exposure Among Children
Younger Than Six Years in the United States)

17. Use of marijuana in adolescence found to increase developing psychosis,
schizophrenia, anxiety, and depression in adulthood.
-- Boston Children’s Hospital/Harvard Medical School, 2014 (Marijuana 101, Dr. Sharon Levy)

18. Cannabis use may cause enduring neuropsychological impairment that persists beyond the period of acute intoxication.
-- Proceedings of the National Academy of Sciences, July 2012 (Persistent Cannabis Users Show Neuropsychological Decline from Childhood to Midlife)

19. Cannabis use disorder is prevalent, associated with comorbidity and disability, and largely untreated.
-- The American Journal of Psychiatry, March 2016 (Prevalence and Correlates of DSM-5 Cannabis Use Disorder, 2012-2013: Findings from the National Epidemiologic Survey on Alcohol and Related Conditions–III)

20. We recorded clear and consistent associations and dose-response relations between the frequency of adolescent cannabis use and all adverse young adult outcomes.
-- The Lancet-Psychiatry, September 2014 (Young Adult Sequelae of Adolescent Cannabis Use: An Integrative Analysis)

21. While marijuana may be safer than alcohol in some respects, there are important dimensions along which marijuana appears to be the riskier substance.
-- Carnegie Mellon Research/Jonathan P. Caulkins, October 2014 (Is Marijuana
Safer than Alcohol? Insights from Users’ Self-Reports)

22. Potential impacts of recreational marijuana include not only increased availability, resulting in ED visits for acute intoxicating effects of marijuana use, but also effects on mental health disorders and psychiatric-related illnesses.
-- American College of Emergency Physicians/ACEP NOW, October 2014 (How Legalizing Marijuana Has Impacted Colorado)

23. Marijuana changes the structure and function of the adolescent brain.
-- Bertha Madras, Professor of Psychobiology, Harvard University, May 2014 (Marijuana and Opioids Risks for the Unborn, the Born)

24. Dramatic increase in newborns testing positive for marijuana in Colorado hospitals.
-- Parkview Medical Center, St. Mary-Corwin Medical Center, Pueblo Community Health Center, April 2016 (Recreational Retail Marijuana Endangers Health of Community & Drains Precious Health Resources)

25. Casual use of marijuana is related to major brain changes.
-- Journal of Neuroscience, April 2014 (Cannabis Use Is Quantitatively Associated with Nucleus Accumbens and Amygdala Abnormalities in Young Adult Recreational Users)

26. It needs to be emphasized that regular cannabis use, defined here as once a week, is
not safe and may result in addiction and neurocognitive damage, especially in youth.
-- Journal of Current Addiction Reports, April 2014 (Considering Cannabis: The Effects of Regular Cannabis Useon Neurocognition in Adolescents and Young Adults)

27. Exposure to cannabis in adolescence is associated with a risk for later psychotic disorder in adulthood.
-- Journal of Current Addiction Reports, June 2014 (Impact of Cannabis Use on the Development of Psychotic Disorders)

28. Marijuana is not benign and there's a mountain of scientific evidence, compiled over nearly 30 years, to prove it poses serious risks, particularly for developing brains.
-- Diane McIntosh, Professor of Psychiatry-University of British Columbia, April 2016 (You Can't Deny Marijuana Is Dangerous For Developing Minds)

28. Marijuana may actually worsen PTSD symptoms or nullify the benefits of specialized, intensive treatment. Cessation or prevention of use may be an important goal of treatment.
-- Journal of Clinical Psychiatry, September 2015 (Marijuana Use is Associated With Worse Outcomes in Symptom Severity and Violent Behavior in Patients With Posttraumatic Stress Disorder)

29. Converging epidemiological data indicate that adolescent cannabis abusers are more likely to develop psychosis and PFC-related cognitive impairments later in life.
-- Journal of Molecular Psychiatry, March 2014 (CB1 Cannabinoid Receptor Stimulation During Adolescence Impairs the Maturation of GABA Function in the Adult Rat Prefrontal Cortex)

30. Regular cannabis use in adolescence approximately doubles the risk of being diagnosed with schizophrenia or reporting psychotic symptoms in adulthood.
-- Journal of Addiction, January 2015 (What Has Research Over the Past Two Decades Revealed About the Adverse Health Effects of Recreational Cannabis Use)
 

Credit to Arizonans for Responsible Drug Policy for this list. You can find out more about their organization by going to www.ardp.org