Cannabis is the most commonly used illicit drug, with 2.5% of the world population—approximately 147 million people—consuming it regularly1. As a consequence, it has become a fixture in medical research, under constant intense scrutiny regarding its beneficial and noxious effects, and its potential medicinal uses. Unfortunately, this often results in an endless barrage of medical news with questionable sources2. Here, we summarize key veritable facts about the effects of cannabis on health.
Prevalence of Cannabis Use and Abuse
In addition to being the most frequently used illicit drug overall, cannabis is also the most common illicit drug consumed by younger populations, typically beginning in adolescence and peaking in early adulthood3. In general, cannabis users do not appear to have significantly higher general mortality rates4; however, it has been consistently linked with increased mortality associated with motor vehicle accidents and lung cancer5.
On the other hand, the relationship of cannabis with mental health is controversial. Although it does not appear to be conclusively related to long-term psychotic disorders or cognitive decline6, dependence remains a major issue. Roughly 13.1 people have been estimated to suffer from cannabis use disorder globally7. Furthermore, this disorder affects 4-8% of adults during their lifetime, and the risk of developing dependence has been estimated at 9% for all subjects who have ever consumed this drug3.
Cannabis abuse is also related to a high prevalence of other mental health problems: Mood disorders have been reported to occur in approximately 45.7% of outpatients with cannabis dependence, while personality disorders and anxiety disorders appear to occur in 21% and 19%, respectively8. Moreover, suicide risk is another major concern in the management of cannabis abuse, especially in chronic cases. Nevertheless, research remains inconclusive in this respect9.
Indeed, cannabis dependence is currently an important public health problem, as it heavily impacts quality of life. At present, cannabis dependence is associated with over 2 million disability-adjusted life years, and is expected to rise in this respect in future years7. Subjects with cannabis dependence also represent increased economic costs for healthcare systems, with a large disease burden10.
Acute Effects of Cannabis
Intoxication with cannabis causes various mental and bodily symptoms which have variable durations depending on the method of consumption. The mental effects are usually the most overt, including euphoria, changes in perception, impaired attention, concentration and short-term memory, slower reaction times, and increased impulsivity, risk-taking and appetite18. Occasionally, unpleasant symptoms such as anxiety, irritability, paranoia and psychosis may occur. These are most frequent in susceptible individuals and first-time users19.
The bodily effects of cannabis intoxication include increased heart rate and breathing, low blood pressure, dry mouth, red eyes, and difficulty with movement coordination20. Death associated with cannabis intoxication is rare, and is usually linked to worsening of preexisting medical conditions, combination with other drugs of abuse, or most frequently, motor vehicle accidents4. Up to 4.2% of all deaths in motor vehicle accidents may be attributed to driving under the influence of cannabis21.
Long-Term Effects of Cannabis
The impact of chronic cannabis use on overall health is one of the most controversial fields of research in modern medicine. Currently available data on this issue is heavily influenced by factors such as ineffective study designs, the concurrent use of other drugs, and a failure to distinguish between the effects on heavy chronic users and light, occasional users22.
Therefore, most evidence to date is inconclusive23. However, research in this field is in constant development, and certain findings offer useful insight into the true long-term effects of cannabis, and its potential therapeutic applications.
Brain and Mental Health
The effects of cannabis on brain structure and function have been extensively studied through various high-technology neuroimaging methods. Regular use of cannabis, especially by smoking, has been associated with changes in the structure of various brain areas, such as the amygdala and hippocampus, as well changes in white and grey matter volume. Likewise, chronic cannabis use has been related to alterations in the metabolism of glutamate, dopamine, and other neurotransmitters24.
In turn, these changes may explain the onset of neuropsychiatric disorders associated with cannabis, besides dependence. This relationship appears to be strongest with psychosis. Cannabis use has been consistently linked with earlier onset of psychotic disorders in many large studies25–27. Furthermore, this relationship appears to be dose-dependent—higher doses are associated with increased effects—and most severe in subjects with cannabis dependence. Continued cannabis use has also been related to increased relapse in psychotic disorders, along with more severe symptoms, decreased level of functioning, and longer hospital stays26.
In contrast with psychosis, the link between cannabis and other mental disorders remains less clear. Regarding cognitive function, chronic use of cannabis has been related to impairment of concentration, memory, and movement, which may persist in the long term even after discontinuation of cannabis28,29. However, currently available systematic reviews on the matter cannot completely attribute these alterations to cannabis, and further research is required.
Lastly, although cannabis is undoubtedly linked with euphoria and decreased anxiety during acute intoxication18, it may promote the development of anxiety and depressive disorders in the long term. In large studies conducted on the general population, chronic cannabis use has been positively associated with anxiety and depressive disorders; although these effects may be limited to heavy users only30,31.
The effects of cannabis on the cardiovascular system are remarkably mixed. Cannabis has been associated with both high and low blood pressure during acute intoxication, which may also trigger a heart attack or stroke32. Most strikingly, the risk of heart attack is 4.8 times higher 60 minutes after the consumption of cannabis33.
However, cannabis may also yield some benefits for cardiovascular on the longer term by alleviating chronic inflammation34. Moreover, cannabis does not appear to be related to several metabolic disorders associated with increased cardiovascular risk, such as insulin resistance and fatty liver disease35. Ultimately, currently available evidence on the link between cannabis use and cardiovascular disease has been deemed insufficient to establish definitive conclusions36.
Chronic cannabis smoking appears to be closely related with lung disease. In a review by Ribeiro and Ind37, cannabis smoking was linked with chronic cough and wheezing, as well as alterations in lung function tests. Moreover, these effects may be reversible upon quitting, and may worsen those caused by tobacco smoking.
Indeed, these effects have been attributed to the harmful impact of smoke on the lungs, rather than cannabis itself. The harmful effects of tobacco and cannabis smoking appear to be difficult to differentiate38. In contrast, although evidence is scarce, edibles and vaporizing do not appear to affect the lungs. Switching to vaporizing from smoking during one month has been reported to improve respiratory symptoms by up to 73%39.
Most alarmingly, smoking cannabis has been related to lung cancer. In a 40-year cohort study, heavy use of smoked cannabis was associated with a more than twofold risk of developing lung cancer40. Nonetheless, it should be noted that large analyses tend to limit this risk to heavy users only, and most frequently in concurrent tobacco users41. Further research is required to clear this issue.
The relationship of cannabis with cancer is a particularly contentious topic, as this drug has been heralded as both a risk factor and a revolutionary treatment alternative for this disease. As a risk factor, cannabis has been consistently linked with lung cancer41; although part of this risk may be due to the concomitant use of tobacco40. Evidence on the link between cannabis and other types of cancer is notoriously insufficient, only suggesting a possible weak relationship with testicular cancer42.
In contrast, cannabinoids have been observed to trigger several anti-cancer effects. These include the modulation of cell growth proliferation and reproduction, as well as inhibition of metastasis43. In addition, other molecules found in cannabis, such as eicosanoids and terpenes, can trigger anti-inflammatory and immunomodulatory effects which may potentiate the action of other anti-cancer drugs44. However, these effects have been mostly observed in vitro, and further investigation is required to assess their clinical relevance43,44.
On the other hand, cannabis has yielded very promising results in the management of the symptoms associated with cancer, especially pain. THC appears to significantly alleviate cancer-related pain, in a dose-dependent manner, and is only rarely associated with adverse effects such as drowsiness, nausea and vomiting45.
Cannabis has been reported to ameliorate other symptoms associated with cancer, such as anorexia, depression, anxiety, weakness and fatigue46. Notably, up to 95.9% of patients with cancer receiving medical cannabis report significant subjective improvement of their conditions after 6 months of use, while 69.5% report a significant improvement in their quality of life47. These results have ignited abundant research on synthetic cannabinoid analogues, which may represent valuable novel agents for the management of chronic pain48.
In addition to their role as analgesic drugs for cancer-related pain, cannabinoids also appear to be useful in the context of other types of pain. Cannabis may alleviate pain not only by binding to receptors in the central and peripheral nervous systems, but also by exerting anti-inflammatory effects and acting in synergy with other analgesics, such as opioids49. However, cannabis does not appear to be equally effective for all kinds of pain, and it appears to be inadequate for the management of acute postoperative pain, abdominal chronic pain, and pain associated with rheumatoid diseases50.
Indeed, a large analysis by Aviram and Samuelly-Leichtag51 suggested cannabis to be effective in the management of non-cancer pain, especially neuropathic pain (i.e. pain caused by nerve damage). However, results are far from uniform, with another analyses reporting these effects to be only mild, with limited impact on other related aspects, such as quality of life and perceived improvement52. At any rate, cannabis appears to be safe in the management of chronic pain, although further research is needed to clarify its true value in clinical practice51.
As continued use of cannabis has become progressively widespread, concerns on its effects on pregnancy and newborn health have increased. Cannabis use during pregnancy appears to be most frequent in women with anxiety and depression53. THC has been proved to cross the placenta, and although fetuses tend to have lower exposure to THC than mothers, it has potential neurotoxic effects for the fetus54.
The impact of cannabis use on pregnancy outcomes remains similarly unclear. In a retrospective study, cannabis alone did not appear to be related to adverse outcomes; rather, concurrent tobacco smoking and other related factors seemed to increase the risk of preterm birth, decreased birthweight, and other complications at birth55. On the other hand, other reports appear more alarming, with exposure to cannabis before birth being associated with decreased birthweight and need for placement in newborn intensive care units56. Research on the effects of cannabis on pregnancy remains in its infancy, and these issues should be clarified in the future.
Although research regarding the health effects of cannabis has skyrocketed in recent decades, many questions remain unanswered. This area of investigation faces several obstacles and challenges, including ethical and legal issues, as well as factors such as the high frequency of concurrent use of other substances, and variations in dosing, methods of administration, and preparations22. Therefore, future research should focus on addressing these situations in order to provide a clearer picture of the impact of cannabis on health and well-being.
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