Relying on NIH studies in this instance is  problematic for a few reasons.  One, funding is only available to study deficits/potential harm, and is not balanced to study possible benefits.  Two, politics affect who/what is studied and how it is reported.  These problems with NIH funding and reporting are widely known among researchers. NIH was intended to be separate from politics but it certainly is not.

Still...

First, take any effects from smoking off the table - they're irrelevant to the thread here, which is focused on vaporizing. Even so, the evidence is mixed and speculative, as indicated in the post. At most, you see missing a day of work here or there and speculation about how pot might cause cancer, but not evidence that it does.

Second, withdrawal symptoms are minor, short-lived (peak at 2-3 days) and it's a red herring to equate them with significant addiction - done for financial (get your funding for research/treatment/enforcement/incarceration), moralistic (drugs are bad and pot is a drug so it must be bad) or propagandistic (pretty much the entire history of pot becoming illegal in the US) reasons.  Quit having sex, quit running, quit anything you like and do regularly and you are likely to have irritability and craving, some distress/anxiety and sleeplessness. If you're a regular meat-eater, just quit that for a week, then report back on your experiences.

Next, the associations with mental illness are only associations.  People get high to cope with anxiety, depression, etc., and the only evidence around schizophrenia is that people who are developing it may have first symptoms appear sooner or more flagrantly if they're also getting high.

Racing heart is a physical effect, not damage or illness.

Perceptual, mood and physical alterations are the reasons people get high.  Calling them distortions makes them sound bad.

How Does Marijuana Affect the Brain?

Scientists have learned a great deal about how THC acts in the brain to produce its many effects. When someone smokes marijuana, THC rapidly passes from the lungs into the bloodstream, which carries the chemical to the brain and other organs throughout the body.

THC acts upon specific sites in the brain, called cannabinoid receptors, kicking off a series of cellular reactions that ultimately lead to the "high" that users experience when they smoke marijuana. Some brain areas have many cannabinoid receptors; others have few or none. The highest density of cannabinoid receptors are found in parts of the brain that influence pleasure, memory, thoughts, concentration, sensory and time perception, and coordinated movement.1

Not surprisingly, marijuana intoxication can cause distorted perceptions, impaired coordination, difficulty in thinking and problem solving, and problems with learning and memory. Research has shown that marijuana's adverse impact on learning and memory can last for days or weeks after the acute effects of the drug wear off.2 As a result, someone who smokes marijuana every day may be functioning at a suboptimal intellectual level all of the time.

Research on the long-term effects of marijuana abuse indicates some changes in the brain similar to those seen after long-term abuse of other major drugs. For example, cannabinoid withdrawal in chronically exposed animals leads to an increase in the activation of the stress-response system3 and changes in the activity of nerve cells containing dopamine.4 Dopamine neurons are involved in the regulation of motivation and reward, and are directly or indirectly affected by all drugs of abuse.

Addictive Potential
Long-term marijuana abuse can lead to addiction; that is, compulsive drug seeking and abuse despite its known harmful effects upon social functioning in the context of family, school, work, and recreational activities. Long-term marijuana abusers trying to quit report irritability, sleeplessness, decreased appetite, anxiety, and drug craving, all of which make it difficult to quit. These withdrawal symptoms begin within about 1 day following abstinence, peak at 2-3 days, and subside within 1 or 2 weeks following drug cessation.5

Marijuana and Mental Health
A number of studies have shown an association between chronic marijuana use and increased rates of anxiety, depression, suicidal ideation, and schizophrenia. Some of these studies have shown age at first use to be a factor, where early use is a marker of vulnerability to later problems. However, at this time, it not clear whether marijuana use causes mental problems, exacerbates them, or is used in attempt to self-medicate symptoms already in existence. Chronic marijuana use, especially in a very young person, may also be a marker of risk for mental illnesses, including addiction, stemming from genetic or environmental vulnerabilities, such as early exposure to stress or violence. At the present time, the strongest evidence links marijuana use and schizophrenia and/or related disorders.6 High doses of marijuana can produce an acute psychotic reaction; in addition, use of the drug may trigger the onset or relapse of schizophrenia in vulnerable individuals.

What Other Adverse Effect Does Marijuana Have on Health?

Effects on the Heart
Marijuana increases heart rate by 20-100 percent shortly after smoking; this effect can last up to 3 hours. In one study, it was estimated that marijuana users have a 4.8-fold increase in the risk of heart attack in the first hour after smoking the drug.7 This may be due to the increased heart rate as well as effects of marijuana on heart rhythms, causing palpitations and arrhythmias. This risk may be greater in aging populations or those with cardiac vulnerabilities.

Effects on the Lungs
Numerous studies have shown marijuana smoke to contain carcinogens and to be an irritant to the lungs. In fact, marijuana smoke contains 50-70 percent more carcinogenic hydrocarbons than does tobacco smoke. Marijuana users usually inhale more deeply and hold their breath longer than tobacco smokers do, which further increase the lungs' exposure to carcinogenic smoke. Marijuana smokers show dysregulated growth of epithelial cells in their lung tissue, which could lead to cancer;8 however, a recent case-controlled study found no positive associations between marijuana use and lung, upper respiratory, or upper digestive tract cancers.9 Thus, the link between marijuana smoking and these cancers remains unsubstantiated at this time.

Nonetheless, marijuana smokers can have many of the same respiratory problems as tobacco smokers, such as daily cough and phlegm production, more frequent acute chest illness, and a heightened risk of lung infections. A study of 450 individuals found that people who smoke marijuana frequently but do not smoke tobacco have more health problems and miss more days of work than nonsmokers.10 Many of the extra sick days among the marijuana smokers in the study were for respiratory illnesses.

Effects on Daily Life
Research clearly demonstrates that marijuana has the potential to cause problems in daily life or make a person's existing problems worse. In one study, heavy marijuana abusers reported that the drug impaired several important measures of life achievement including physical and mental health, cognitive abilities, social life, and career status.11 Several studies associate workers' marijuana smoking with increased absences, tardiness, accidents, workers' compensation claims, and job turnover.

Revised 7/09