The general consensus is that cannabis is generally safe when used in a controlled manner. However, there remain a number of risk factors that prevent it from being completely safe, much in the same way that alcohol, a legal and widely used drug, is not considered to be completely safe as well.
Here are some of the more important risk factors to consider when using cannabis for medical or recreational purposes.

Impaired driving

Cannabis compounds have been linked to drastically slowed reaction times and impaired decision-making. States that have legalized cannabis have experienced a jump in cannabis-related vehicular accidents. Massachusetts, for instance, reported a 174.5% relative increase in cannabis-related accidents after decriminalization.

On the other hand, data from the US Federal Government shows that, in relative terms, cannabis-impaired drivers are still safer than drunk drivers. However, both types of drivers are still significantly less safe than sober drivers, which makes it important to ensure that one avoids cannabis before driving or operating heavy machinery.

Adolescent brain development

If there are any controls on recreational marijuana or cannabis use, it may very well make sense to have a relatively high age limit. While generally safe for adult brains, multiple studies indicate that cannabis can have very serious effects on developing adolescent brains, negatively affecting development and severely hampering cognition, learning, and memory in adulthood.

While the legal age of adulthood in most US states is 18, the fact is there is tremendous variance in the development of adolescent brains, and brains can become “adult” as late as 25. The evidence seems to show that heavy or even mild frequent use may irreversibly damage adolescent brains.

Even if the damage turns out not to be permanent, habitual cannabis use in teens has also been linked with lower achievement in school, which also brings a whole host of other problems down the road in term of social and professional opportunities as well.

Psychological dependence

It’s generally acknowledged that uncontrolled cannabis use can lead to psychological dependence. Similar to other types of psychological dependencies, such as gambling, psychological dependence on cannabis can lead to a drastically reduced quality of life. This dependence can lead to a loss of social and work opportunities, create a financial strain for the user, and lead to social isolation in extreme cases.

The risk of dependence lies in a wide number of factors, including genetics. This means that a predisposition towards psychological addiction can run in families. This means that if you have parents who have the disposition for dependencies of any kind, you likewise have an elevated risk for psychological dependence on cannabis.

Schizophrenia and psychosis

There is now a definite link between cannabis and a number of mental disorders, most notably schizophrenia and psychosis.

A strong link to schizophrenia has been noted in a wide number of studies. This does not definitively mean that cannabis use causes these disorders. It’s possible that people with schizophrenia are just more likely to seek out cannabis and habitually use it, rather than cannabis causing the condition by itself. But due to the difficulty of conducting these studies, the risks cannot yet be ruled out.

Cannabis, on the other hand, has been definitely shown to cause psychosis. However, it must be noted that caffeine, alcohol, nicotine, and “magic mushrooms” may also cause psychosis as well. The risks are can also be mitigated if cannabis is used in controlled conditions.

Thanks to the efforts of researchers, the risks of medical cannabis are quite low, and in many cases lower than some mainstream treatments such as opioids. This is especially true when products such as CBD oil are used instead of traditional THC-based medications.