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I've heard a lot of conflicting information about the long-term effects. Could someone please clarify at least the proven risks?

Note: I've decided to answer my own question, but I'd prefer to hear from others too. Following the guidelines of this site, any academic studies related to this question are welcome and encouraged, NOT pure opinion or anecdotal evidence.

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    I feel the need to include the note as a reminder in case people have strong opinions on this subject. – Dave Liu Apr 17 '15 at 22:14
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    Driving while drugged can definitely be hazardous to health. – Iron Pillow Apr 27 '15 at 4:44
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This seems like a difficult question as there continues to be a large amount of conflicting reports about the extent of biological consequences regarding cannabis usage.

However,

  • There is general consensus that smoking cannabis causes irreversible cognitive impairment in children and pre-pubescent adolescents. {6}
  • Marijuana smoke has been listed on the California Proposition 65
    warning list as a carcinogen since 2009. {7} One should note this is the smoke, not the plant.
  • Cannabis consumption in pregnancy is associated with restrictions in growth of the fetus, miscarriage, and cognitive deficits in offspring. {8}

  • Contrary to common belief, marijuana can be addictive. Research suggests that about 1 in 11 users becomes addicted to marijuana (Anthony, 1994; Lopez-Quintero 2011).This number increases among those who start as teens (to about 17 percent, or 1 in 6) and among people who use marijuana daily (to 25-50 percent) (Hall, 2009a; Hall, 2009b).{9}

Given the above references, there are some points that should be emphasized.

  • Marijuana use is not associated with elevated cancer risk as shown in preclinical studies.{10} (Separate link to news article here.)
  • [Marijuana does not cause long-term cognitive impairment in users who start after 21 years old.{12}
  • Compared to other substances, marijuana is not very addicting. It is estimated that 32% of tobacco users will become addicted, 23% of heroin users, 17% of cocaine users, and 15% of alcohol users. Cocaine and heroin are more physically harmful and nicotine is much more addictive. It is much harder to quit smoking cigarettes than it is to quit smoking pot.

There are some studies that indicate "marijuana may increase cravings for other drugs" and that "THC exposure increases tobacco’s addictive effects", which is what leads to the Gateway Drug Theory. This is the theory that marijuana causes an individual to become addicted to other drugs more easily. Although not fully proven, these sources indicate some correlations when tested on mice, meaning people who use marijuana may be more likely to get addicted to cocaine, heroin, and other similar heavily addicting drugs. This sounds frightening except that the majority of people don't try those more dangerous drugs.

Boiling it all down, these sources seem to suggest that marijuana is NOT a carcinogen if you don't smoke it, but does cause mental and physical detriments to anyone still growing. There's a small chance you could get addicted if you use it as a coping mechanism, but statistically speaking, there's a less than 10% chance of getting addicted. Additionally, if you decide to use marijuana, don't try other drugs that could be addictive.

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