Marijuana, the most commonly used illegal drug, is a combination of shredded leaves, stems and flower buds of the Cannabis sativa plant. It can be smoked, eaten, vaporized, brewed and even taken topically, but most people smoke it. Contrary to popular belief, marijuana is a dangerous drug and continued use can have serious consequences on the normal functioning of your body.
With all the talk these days about legalizing marijuana, it can be hard to sift through all the information that’s out there. No doubt at some point you will be given an opportunity to try it while being told that it’s no big deal and won’t do any harm. However, make sure you have all the facts so you are able to make an informed decision.
A few things to ponder
It’s addictive. Contrary to popular belief, marijuana is an addictive drug. One in six youth who try it become addicted, while one in 10 adults who try it become addicted.
It’s been genetically modified. The cannabis plant of today is not what Mother Nature intended. In the 1960s and ‘70s, THC levels of marijuana averaged around 1%, increasing to just under 4% in 1983, and almost tripling again in the subsequent 30 years to around 11% in 2011. THC is the psychoactive ingredient in marijuana, and the high potency levels found in modern marijuana can result in experiencing psychosis. In the meantime, CBD (Cannabidiol), which counter acts the anxiety-inducing properties of THC, remains the same.
It impairs how your brain and body work together. Everything slows down when you smoke pot. That might feel good when you’re trying to escape the stresses of your life, but you can put yourself and others at risk when you are functioning at a slower level. For example, since becoming legal in Colorado, fatal car accidents involving drivers testing positive for marijuana rose by 112%. 
It can change your brain. Studies have shown that using marijuana before the age of 21 can significantly change brain structures and functioning. Youth with poor academic results are more than four times likely to have used marijuana in the past year than youth with an average of higher grades.  Frequent use, especially starting around 13, is associated with memory trouble, not doing as well in school, and impaired thinking. One study that followed people for 20 years found that those who had smoked frequently in their youth had an average IQ of 8 points lower, even after they had quit.
It can increase depression and anxiety. You might start smoking pot because you believe it will help you deal with anxiety and depression. In truth, it is not long before it exacerbates these conditions resulting in cannabis use disorder (CUD), anxiety/mood disorders, cognitive dysfunction, psychosis, and schizophrenia. If you or a family member has a history of mental illness, you’re also at an increased risk of developing psychosis.
It can negatively impact your physical health. Using pot has been shown to weaken your immune system, leaving you susceptible to all kinds of respiratory problem including increased coughing, phlegm, wheezing, and infection. For guys, the risk for testicular cancer increases by 70 percent, and not only that, it has been shown to cause sexual impotency. 
It can be laced with pesticides. There are no standards for growing marijuana, so many of the chemicals applied to pot plants are intended only for lawns and other non-edibles. Medical cannabis samples collected in Los Angeles have been found to contain pesticide residues at levels 1600 times the legal digestible amount. 
It kills wildlife and harms the environment. Something you might not know is that environment researchers have found that pot farms in California are endangering wildlife such as salmon and black bears. “Farmers” have illegally mowed down trees, graded hilltops flat for sprawling greenhouses, dispersed poisons and pesticides, drained streams, and polluted watersheds. 
It can be a gateway to other drugs. There is a consistent association found between using marijuana and progression to the use of other substances.
A closer look -- your brain and marijuana
Let’s take a closer look at what happens when you take a hit.
THC enters your bloodstream and crosses into your brain, binding to cells containing cannabinoid receptors. These receptors are part of the endocannabiniod system, which helps us with memory, thinking, learning, balance, and concentration. Activating this system causes your brain to release dopamine, giving you that “high” feeling. 
Some users experience heightened sensory perception, with colors appearing more vivid and noises being louder. For some, marijuana can cause an altered perception of time and an increased appetite, or the “munchies.”
The impact of using marijuana can vary by person, how often they have used the drug, the strength of the drug, and how often it has been since they have gotten high, among other factors.
Other effects, according to the American National Institute of Health (NIH), include:
Feelings of panic, anxiety and fear (paranoia)
Increased heart rate
Decreased ability to perform tasks that require coordination
Decreased interest in completing tasks
When coming down from the high, users may feel depressed or extremely tired. While marijuana use produces a mellow experience for some, it can also heighten agitation, anxiety, insomnia and irritability, according to the NIH.
Marijuana doubles the risk of car crash.
Marijuana reaches the same pleasure centers in the brain that are targeted by heroin, cocaine and alcohol.
Marijuana disrupts short-term memory but not long-term memory. This effect occurs because of marijuana’s impact on the hippocampus, that part of the brain responsible for memory formation. The hippocampus normally loses neurons due to aging. Chronic marijuana use seems to increase that loss.
Marijuana may affect certain neurons in the brain that are normally responsible for suppressing appetite, and this effect may explain why people often get very hungry after smoking pot, according to a 2015 study in mice.
A growing number of studies show that regular marijuana use ( once a week or more) actually changes the structure of the teenage brain, making it harder for teens to cope with social situations and the normal pressures of life.
Studies done on lab rats suggests cognitive deficits linked to marijuana use may be long-term. Even after the equivalent of nine human years without marijuana exposure, rats given marijuana extract in adolescence showed residual mental deficits in learning and memory that persisted into adulthood. But rats given marijuana extract as young adults did not develop long-lasting impairments. The researcher says that cannabis receptors in the brains of humans and rodents work “in very similar ways.” 
Sabet, Kevin A. Reefer Sanity: Seven Great Myths about Marijuana. New York, NY: Beaufort, 2013.
National Cannabis Prevention and Information Centre. Cannabiniods; Research Brief; 2011.
Iseger, T.; Bossong, M. A Systematic Review Of The Antipsychotic Properties Of Cannabidiol In Humans. Schizophrenia Research 2015, 162, 153-161.
"Marijuana-Related Fatal Vehicle Accidents Increase 112 Percent in Six Years in Colorado." Institute for a Drug Free Workplace, 4 July 2014. Web
Moore, T.; Zammit, S.; Lingford-Hughes, A.; Barnes, T.; Jones, P.; Burke, M.; Lewis, G. Cannabis Use And Risk Of Psychotic Or Affective Mental Health Outcomes: A Systematic Review. The Lancet 2007, 370, 319-328.
Canadian Centre on Substance Abuse,. Chronic Use And Cognitive Functioning And Mental Health; Clearing the Smoke on Cannabis; Canadian Centre on Substance Abuse: Ottawa, 2009.
Meier, M.; Caspi, A.; Ambler, A.; Harrington, H.; Houts, R.; Keefe, R.; McDonald, K.; Ward, A.; Poulton, R.; Moffitt, T. Persistent Cannabis Users Show Neuropsychological Decline From Childhood To Midlife. Proceedings of the National Academy of Sciences 2012, 109, E2657-E2664.
Pletcher, M.; Vittinghoff, E.; Kalhan, R.; Richman, J.; Safford, M.; Sidney, S.; Lin, F.; Kertesz, S. Association Between Marijuana Exposure And Pulmonary Function Over 20 Years. JAMA 2012, 307, 173.
"Marijuana Linked With Testicular Cancer." National Institute on Drug Abuse (NIDA). N.p., 01 Dec. 2010. Web.
"Is There a Link between Marijuana Use and Psychiatric Disorders?" | National Institute on Drug Abuse (NIDA). Web.
Peeples, Lynne. "Marijuana Pesticide Contamination Becomes Health Concern As Legalization Spreads." The Huffington Post. TheHuffingtonPost.com, 24 May 2013. Web.
"Toke This: The Unexpected Effect of California's Pot Farm Explosion on Wildlife." TakePart. 26 July 2013. Web.
Fergusson, D.; Boden, J.; Horwood, L. Cannabis Use And Other Illicit Drug Use: Testing The Cannabis Gateway Hypothesis. Addiction 2006, 101, 556-569.
Niesink, R.; van Laar, M. Does Cannabidiol Protect Against Adverse Psychological Effects Of THC?. Front. Psychiatry 2013, 4.
Bostwick, J. Blurred Boundaries: The Therapeutics And Politics Of Medical Marijuana. Mayo Clinic Proceedings 2012, 87, 172-186.
Hartman, R.; Huestis, M. Cannabis Effects On Driving Skills. Clinical Chemistry 2012, 59, 478-492.
Inaba, Darryl and William E. Cohen. Uppers, Downers, All Arounders: Physical and Mental Effects of Psychoactive Drugs. Ashland, Or.: CNS Publications, 2000. 251-252.
Blaszczak-Boxe, Agata. "Marijuana Munchies May Come from Scrambled Neuron Signals." Live Science. N.p., 18 Feb. 2015. Web.
"Behavior & the Teen Brain." Partnership for Drug-Free Kids. Web.
Quinn, Heidi R., Izuru Matsumoto2, an Paul Callaghan, et al. "Adolescent Rats Find Repeated D9-THC Less Aversive Than Adult Rats but Display Greater Residual Cognitive Deficits and Changes in Hippocampal Protein Expression Following Exposure." Neuropsychopharmacology 33(5):1113-26 · May 2008