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The safety of cannabis has been confirmed when comparing it to other psychoactive substances. The risk of exposure to cannabis is approximately 100 times lower than other drugs like alcohol, opiates, cocaine, or amphetamines. Nevertheless, just because cannabis’ dangers are lower does not mean it is entirely free from potential adverse consequences.
Physiological Response of Cannabis Intoxication
The two primary cannabinoids, tetrahydrocannabinol (THC) and cannabidiol (CBD), interact with two distinct cannabinoid receptors within the body, called CB1 and CB2. 
THC interacts primarily with CB1 receptors, which tend to be concentrated on nerve cells in the brain and spinal column. This is why the primary intoxication from cannabis tends to be more cerebral, and less physical.
CBD, in contrast, bonds primarily with CB2 receptors, which are found in both the brain and the rest of the body. CBD is not psychoactive, but because of the location of CB2 receptors, it has both neurological and physical side effects. 
With this in mind, it is still possible to experience a cannabis overdose, but a better word may be overconsumption. This is because imbibing enough cannabis to cause death extremely unlikely, but the way cannabis is ingested does play a role in how it is metabolized. 
When inhaled, cannabis is quickly absorbed into the bloodstream, where it can be processed relatively quickly by the liver. Following inhalation, the effects are almost immediate, with a peak lasting for approximately 1-2 hours.
When ingested, cannabis is absorbed through the digestive system and must first go through the liver before entering the bloodstream. Because of this, the onset takes much longer, but it offers a much higher threshold for how many cannabinoids can be absorbed. This is why the risk of discomfort from overconsumption of cannabis is higher when eaten compared to when it is inhaled. 
Symptoms of acute cannabis intoxication can include anxiety, panic attacks, unresponsiveness, increased heart rates, and spikes in blood pressure.  While these symptoms may appear relatively mild in comparison to other overdoses, hospitalizations are possible following overconsumption of cannabis.
Psychological Disorders and Cannabis
More concerning than the short term risks associated with cannabis use are the statistical possibility of developing long term mental disorders from frequent use.
The most troubling is the risk of developing psychosis, specifically in the form of schizophrenia.  The cause of schizophrenia is unknown, so it is impossible to determine the extent that cannabis use may cause the disorder. One study found that cannabis use can increase the risk of developing long term psychosis by as much as 37%, but this number should be taken with caution.  The study evaluated candidates who were already predisposed to developing schizophrenia, and not of the population as a whole. In reality, the current opinion is that most cannabis users are not likely to develop this psychological disorder, but it does have the potential of unlocking it in those who may already be at risk due to genetic or other external factors.
When looking at the general population, only 0.47% of users report developing long term cannabis associated psychotic symptoms.  So while the risk is generally low, it may be higher for certain individuals. Evidence suggests that the probability increases for cannabis users who started using during adolescence, and the odds are also higher for men than for women. 
Beyond psychosis, cannabis users also have the potential to develop anxiety and depression.  While a link does appear to exist, it may be a question of correlation and causation. These conditions only appear to develop with heavy cannabis use, and are rare for those who only consume it occasionally. One theory to account for this is that those who suffer from anxiety or depression may choose cannabis to self-medicate.  This may exacerbate the problem though, as THC can magnify these conditions, while CBD may help temper them. 
Substance Abuse and Long Term Risk
While substance abuse can be classified as a psychological problem, it falls into a slightly different category. As a psychoactive compound, it is possible to become addicted to cannabis, which usually takes the form of psychological addiction. 
Physical withdrawal symptoms are also possible, but they do not affect all users equally.  The longer the addiction persists, the more likely long term health complications are to develop. With sustained cannabis use, the effects can be similar to tobacco smokers. This can take the form of heart disease, lung damage, and possibly cancer. 
These symptoms are all likely related to inhaling smoke into the lungs, but it is also possible for cannabis to impact the gastrointestinal tract as well.
Long term cannabis use may also lead to the development of cannabinoid hyperemesis syndrome (CHS).  The full causes of this syndrome are not fully understood, but it is identified by frequent and intense vomiting.
There are also potential reproductive risks associated with extended cannabis use. For women, it is always a good idea to avoid any psychoactive substances during pregnancy, including cannabis. Evidence has linked cannabis use during pregnancy to lower birth weights and to later cognitive development problems for the child. 
For men, long term cannabis use has been shown to lower sperm counts, and may lead to DNA methylation in sperm as well. 
Finally, there is conflicting evidence about how much cannabis use may alter the brain. One study found that cannabis use early in life may lead to a 6-8 point drop in IQ.  On the other hand, another study conducted on twins where one used cannabis and the other abstained showed no major differences between their brains.  One explanation for this seems to point to cannabis affecting both white matter and hippocampal volume, but more evidence is needed to determine both the link to cannabis and the confirmation of cognitive decline. 
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