The Adverse Effects on Health Caused by Smoking Marijuana

Smoking marijuana has become a controversial national topic. Circumventing the Food and Drug Administration (FDA) drug approval process, a 1996 ballot initiative in California enabled legal access to marijuana for medical purposes. This action was replicated by ballot or legislative initiatives in 23 states and the District of Columbia, culminating in the legalization of marijuana in 2012 by Washington state and Colorado.

The shifting status of marijuana reflects a change in public perception and belief that marijuana is harmless. Advocates claim that smoking marijuana is a harmless activity, while opponents voice deep concerns about its adverse consequences. Paradoxically, public perception of marijuana as a safe drug is rising, at the same time that scientific evidence of marijuana’s harms is rapidly increasing, especially for young people.

It is also important to note that the marijuana plant has been bred to increase the levels of its psychoactive constituent, THC or Δ9-tetrahydrocannbinol. The marijuana of yesteryear contained from 1% to 3% THC, now crops contain from 9% to 20% or more THC. This change is important because the higher the concentration of THC, the more THC enters the brain to produce a higher level of intoxication.

We all recognize that marijuana is a psychoactive drug that is used to engender euphoria (“high”), relaxation, and sedation. Some people do not appreciate its effects because they don’t like to lose control of their thoughts, or don’t enjoy the associated anxiety, stress, and infrequent psychotic symptoms. What are some of its immediate and long-term consequences that are detrimental to the brain, body and behavior?


In the short term, marijuana is an intoxicant that impairs memory, sense of time, and reaction time, how we perceive the world around us (sounds, visuals), attention span, ease of speaking and problem solving, and control over motor function. The effects can last from 1 hour to many hours, depending on its potency, how it is ingested, user experience, or unique individual responses. Regardless, marijuana intoxication can impair brain function (learning and memory) and motor function in driving or other activities requiring motor coordination.

Long-term, heavy marijuana use can impair brain function, with an impact on memory, attention, learning, and motivation. Long-term, frequent marijuana users show a loss of IQ points over time, which can affect their daily lives and employment opportunities. Between 25% and 50% of daily users are at high risk of becoming addicted to marijuana. Long-term users are also at higher risk of developing psychosis, a temporary or lasting condition that prevents separating reality from fantasy.

In the worst-case scenario, marijuana increases the risk for developing schizophrenia, a brain disease characterized by thought disorder, hallucinations, delusions, and other serious malfunctions. It can also exaggerate schizophrenia-like symptoms in people harboring the disorder. Long-term marijuana use is also associated with panic reactions, anxiety or symptoms of depression.

In all these consequences, children and adolescents are more vulnerable to the toxic effects of marijuana on the brain. The combination of alcohol and marijuana elicits an even greater impact.


Marijuana smoking can affect many organs in the body, foremost, the lungs and cardiovascular system. Smoked marijuana and smoked tobacco both release similar amounts of tar and toxic chemicals. Not surprisingly, smoking marijuana can result in inflammation and harm to tubes leading to the lungs (bronchitis), to the cells lining the respiratory tract, with pre-cancerous changes in the lungs. Because marijuana stimulates heart rate and changes blood pressure, there is a higher incidence of heart attacks in young people using marijuana.

Pregnant women should be aware that marijuana constituents can pass through the placenta to the developing fetus. A nursing infant can also absorb them from breast milk. Use of the drug during pregnancy has been linked to low birth weight, developmental delay, and behavioral problems (for example inattention) in later childhood. In men and women, marijuana can affect reproductive systems and function, with uncertain long-term consequences.


For young people at the prime of their educational development, marijuana use is associated with poor school performance and school dropout rates. Several studies on college campuses have shown that heavy marijuana users are less engaged in academics, show up for class fewer days, and are less likely to participate in an array of extracurricular activities that enriches college life. Also, their list of personal, family, and social problems is higher compared with non-using students. These consequences may have enduring effects on employment opportunities and income.