Dependence Rate of Weekly Users
A 2016 study examined weekly users of alcohol, tobacco, marijuana and cocaine. It found that 25% of weekly marijuana users qualified as being dependent, making marijuana less addictive than nicotine (67.3%) and cocaine (49.9%), but more addictive than alcohol (15.6%).
Marijuana Dependence Worldwide
A 2017 World Health Organization report stated that, in 2012-2013, almost 30% of marijuana users had a marijuana-use disorder. Among people who have ever used marijuana, the likelihood of becoming dependent was estimated (in the early 1990s) at 9%.
Globally, about 13.1 million are dependent on marijuana, and marijuana was "second only to alcohol" as the primary reason for entering substance abuse treatment. An estimated 1-2% of adults suffered from a marijuana-use disorder in 2016. Those who have ever had such a disorder in their lifetimes comprise about 4-8% of the world population.
In the US, marijuana use increased substantially between 2001 and 2013, with a corresponding rise in marijuana addiction.
Source: World Health Organization
Dependence and Addiction, Explained
Problem use of marijuana is known as a "marijuana use disorder." This is a physical drug dependence that, when severe enough, rises to the level of an addiction. Substance use studies often measure dependence rather than addiction.
Marijuana dependence means that the brain has adapted to heavy marijuana use by becoming less sensitive to it. When a dependent person stops using marijuana, he will experience withdrawal symptoms:
People who use marijuana frequently often report irritability, mood and sleep difficulties, decreased appetite, cravings, restlessness, and/or various forms of physical discomfort that peak within the first week after quitting and last up to 2 weeks.
Dependence qualifies as addiction when the user cannot stop despite adverse effects of marijuana use on his life.
Source: National Institute on Drug Abuse
Physical Dependence and Addiction
Decline in Crystallized Intelligence
A 2015 paper studied two long-term twin studies, comparing teenagers who used marijuana to their twins, who did not. Various IQ subtests were performed on the subjects. The study found no differences in fluid intelligence, but marijuana users scored lower on tests of crystallized intelligence (the ability to reason using knowledge, skills and experience). The amount of marijuana used did not appear to make a difference; the difference in test scores was correlated with a history of any marijuana use.
Lower IQ Scores for Heavy Teen Users
A long-term study published in 2002 found that teens who used marijuana heavily (at least five joints a week) demonstrated a decrease in IQ score. Light users and former heavy users did not suffer from the same effect. The researchers cautioned that "whether the absence of a residual marijuana effect would also be evident in more specific cognitive domains such as memory and attention remains to be ascertained."
Decline in Neurocognitive Performance
In 2002, a group of Johns Hopkins researchers performed neurocognitive tests on marijuana users who had abstained for four weeks. They observed that heavy marijuana users performed significantly worse:
As joints smoked per week increased, performance decreased on tests measuring memory, executive functioning, psychomotor speed, and manual dexterity.
Performance seemed to be unaffected by the duration of the subjects' marijuana use.
Cerebral Dysfunction in Recovering Teens
A small-scale study was performed in 2008 on 15 adolescents in recovery from marijuana dependence. They had abstained from marijuana for an average of 27 days. While their intelligence measured in the normal range, their memory test scores were significantly reduced in comparison. The authors described these results as being "consistent with the pattern produced by cerebral dysfunction" and "a characteristic finding in patients who are recovering form chronic impairment of memory function."
Changes in Brain Structure
A study of the brain structure of chronic marijuana users found significant differences in the orbitofrontal cortex when compared to controls. One of these differences, increased orbitofrontal connectivity, was related to earlier ages of initial marijuana use. A structural difference was also found between regular users and long-term regular users. The orbitofrontal cortex is associated with decision-making.
Large Study Shows Neuropsychological Decline
One study performed neuropsychological tests on over 1000 subjects during their early adolescence and again 25 years later. It found that "Persistent cannabis use was associated with neuropsychological decline broadly across domains of functioning, even after controlling for years of education." Those who had started using marijuana in their adolescence showed greater impairment and did not fully recover their neuropsychological function even after quitting.
According to a 2016 review of the literature, marijuana users produce less dopamine and also react less to dopamine stimulation. In addition, "there is evidence that long-term heavy cannabis use is associated with educational underachievement and impaired motivation" Studies have not yet shown whether marijuana use causes an amotivational state or is correlated with it in some other way.
Source: JAMA Psychiatry
Marijuana Use Linked to Psychiatric Disorders
Marijuana use can trigger an acute psychotic reaction as one of its less common symptoms. It increases the risk of developing schizophrenia (double the risk for those who have ever used marijuana, and six times the risk for those who use marijuana frequently or at high concentrations). It also worsens the course of schizophrenia for those who already suffer from it.
Marijuana use also increase the risk of dependence on other drugs, including nicotine and alcohol.
Cognitive Development of Three-Year-Olds
A 1994 study of 655 children showed that 3-year-olds whose mothers used marijuana during pregnancy measured significantly worse on the Stanford-Binet Intelligence Scale. For white children, preschool or daycare attendance compensated for this negative effect.
Source: Neurotoxicology and Teratology
Reduced Cognitive Performance in Adolescents
A 2002 study determined that adolescents prenatally exposed to marijuana had normal IQs and basic visuoperceptual skills (visual object recognition). However, they performed worse at "application of these skills in tasks in problem-solving situations requiring visual integration and analytical skills as well as sustained attention."
According to a 1999 study, children prenatally exposed to marijuana showed no size differences at birth compared to children who were not exposed. However, in adolescence, they had smaller head circumferences.
Cannabinoid Hyperemesis Syndrome
Long-term marijuana users can develop a condition called Cannabinoid Hyperemesis Syndrome. This begins with early morning nausea and abdominal pain, a phase which can last for months or years. Eventually, the users suffer intense bouts of vomiting lasting 1-2 days.
The hyperemetic phase is characterized by paroxysms of intense and persistent nausea and vomiting, commonly described as overwhelming and incapacitating. Patients vomit profusely, often without warning and can vomit and retch up to five times per hour.
Abstaining from marijuana for a long enough period of time can alleviate the symptoms of CHS, but returning to marijuana use often triggers a relapse.
After medical marijuana became widely available in Colorado, the rate of emergency room visits due to cyclic vomiting nearly doubled.
Increased Levels of VOCs and PAHs
Polycyclic aromatic hydrocarbons (PAHs) and volatile organic compounds (VOCs) are combustion byproducts that may be toxic. A 2016 study found that marijuana smokers had more PAH and VOC metabolites in their urine than non-users, suggesting that the marijuana users were exposed to higher levels of PAHs and VOCs.
Source: Environment International
Harmful Chemicals in Smoke
Other Health Effects
A 2008 study found that marijuana smoke contained more nitrous oxide, ammonia, hydrogen cyanide, aromatic amines, pyridine, acrylonitrile, toluene and styrene than tobacco smoke. It had the same amounts of chromium, nickel and selenium and almost as much tar and benzene.
The marijuana smoke also contained carbon monoxide, other heavy metals and carbonyl compounds, though in lower amounts than in the tobacco smoke.
Source: Chemical Research in Toxicology
Marijuana Comes with Serious Health Risks
Marijuana use involves many known health risks, particularly to children and adolescents. It can permanently damage neurocognitive functioning and alter brain structure. It can cause psychosis and appears to precipitate schizophrenia in some users. It increases the risk of respiratory problems, heart attack, stroke and cancer. Marijuana is more addictive than alcohol, with an estimated 25-30% of users having some degree of marijuana use disorder.
Today's Higher Concentrations Increase Risk
Today's recreational marijuana contains many times more THC than it had in the 1960s and 1970s. This increase in concentration brings a higher risk of adverse health effects, inadvertent overdoses by users, and accidental poisonings of unwary consumers (particularly children and pets).
Like the alcohol and tobacco industries, the growing marijuana industry will oppose regulation and attempt to gain as many users as possible. While marijuana may be legalized, it is heavily in the public interest to prevent its use from becoming normalized.
Prohibit advertising of marijuana products and dispensaries.
Require marijuana-related businesses to present a discreet appearance with no logo or other branding.
Prohibit dispensaries from carrying any non-marijuana-related products.
Prohibit free giveaways or other promotions of marijuana products.
Continue to prohibit public use, maintaining a broad definition of what is "public."
Avoid overdoses and poisonings:
Place limits on the THC content and concentration of marijuana products.
Require child-proof packaging of all marijuana products.
Require marijuana products and packaging to have a distinctive appearance.
Require marijuana product packaging to display a prominent warning label.