Adverse Effects of Marijuana
Nonmedical use of marijuana continues to be problematic in society. Approximately one third of all Americans over 12 years of age have tried marijuana, usually experimenting first during adolescence.4 However, the age distribution of those who use marijuana among the general population declines sharply after age 34, while most medical marijuana users are more than 35 years of age.4 Since 1997, an increasing percentage of young adults aged 18 to 25 years (up from 12.8% to 16.4%) and a decreasing percentage of youths aged 12 to 17 years (down from 9.4% to 7%) perceive smoking marijuana once or twice a week as presenting great risk of harm.186 It is not clear whether these trends have been influenced by the medical marijuana debate.
Most research on the harmful consequences of marijuana use has been conducted in simulated laboratory environments and in individuals who use cannabis for nonmedical purposes. Some of marijuana’s adverse effects differ in experienced versus inexperienced users, and it is not clear to what extent the adverse effects reported in recreational users are applicable to those who use medical marijuana.
Acutely, marijuana increases heart rate, and blood pressure may decrease on standing. Intoxication is associated with impairment of short-term memory, attention, motor skills, reaction time, and the organization and integration of complex information.187,188 Although dependent on the setting, marijuana can cause relaxation and enhance mood. Ordinary sensory experiences may be intensified, with increased talkativeness, perceptual alterations, and distortion in time sense followed by drowsiness and lethargy. These effects appear to be mediated by CB1 receptors because they are diminished by selective antagonism of the CB1 receptor.189 However, some individuals experience acute anxiety or panic reactions, confusion, dysphoria, paranoia, and psychotic symptoms (eg, delusions, hallucinations).188
Heavy users may experience apathy, lowered motivation, and impaired cognitive performance.188 Chronic marijuana use is associated with development of tolerance to some effects and the appearance of withdrawal symptoms (restlessness, irritability, mild agitation, insomnia, sleep disturbances, nausea, cramping) with the onset of abstinence. Depending on the measures and age group studied, 4% to 9% of marijuana users fulfill diagnostic criteria for substance dependence. Although some marijuana users develop dependence, they appear to be less likely to do so than users of alcohol and nicotine, and the abstinence syndrome is less severe.4,188,190 Like other drugs, dependence is more likely to occur in individuals with co-morbid psychiatric conditions.
Data on drug use progression and the view that marijuana is a "gateway" drug pertain to nonmedical use. However, "present data on drug use progression neither support nor refute the suggestions that medical availability [of marijuana] would increase drug abuse."4 Of comparable or greater concern, are potential adverse effects of cannabinoids and marijuana smoke on the immune, respiratory, cardiovascular, and reproductive systems, and the potential for enhancing carcinogenesis."4
In addition to effects attributable to THC, the chronic effects of marijuana smoke are of perhaps greater concern than marijuana’s acute safety profile. Like tobacco, chronic marijuana smoking is associated with lung damage, increased symptoms of chronic bronchitis, and possibly increased risk of lung cancer.
Cannabinoids impair cell-mediated and humoral immunity in rodents and decrease resistance to infection; however, there is no conclusive evidence that consumption of cannabinoids impairs human immune function, and some evidence suggests that cannabinoids exert anti-tumor effects in human cancer cell lines.191,192 Cannabinoids and cannabinoid receptor agonists exert both inflammatory and anti-inflammatory effects and also appear to modulate hematopoiesis and possibly tumor growth.193 Clearly, cannabinoids are immune modulators, but how they regulate various elements of the human immune response is unclear.194 The initial safety trials in HIV-positive patients are encouraging regarding a lack of apparent marijuana-induced immunosuppression, but these were short-term trials