Quote Originally Posted by FindingTim
However, I believe that excessive use causes health problems and is natures way of saying, "enjoy me, but don't abuse me." The way marijuana makes the mind expand and lets us appreciate life is so phenomenal that I simply cannot believe that it was not meant to be consumed.
Heavy use has been shown to affect those under twenty years of age. However, these studies may change your mind as to whether it harms adults, or not.

Lack of hippocampal volume change in long-term heavy cannabis users.
http://marijuana.researchtoday.net/archive/2/4/358.htm
We used magnetic resonance imaging to investigate these effects in a group of 22 older, long-term cannabis users (reporting a mean [SD] of 20,100 [13,900] lifetime episodes of smoking) and 26 comparison subjects with no history of cannabis abuse or dependence. When compared to control subjects, smokers displayed no significant adjusted differences in volumes of gray matter, white matter, cerebrospinal fluid, or left and right hippocampus.
Minimal Long-term Effects Of Marijuana Use Found In Central Nervous System By UCSD Researchers
http://www.sciencedaily.com/releases...0630112652.htm
In addition, Grant said that heavy marijuana users often abuse other drugs, such as alcohol and amphetamines, which also might have long-term neurological effects. This raises the question of the extent to which the other drugs contributed to the minimal problems found in learning and forgetting in the marijuana users.
Non-acute (residual) neurocognitive effects of cannabis
use: A meta-analytic study
http://www.hnrc.ucsd.edu/publication...348art2003.pdf
The small magnitude of effect sizes from observations of chronic users of cannabis suggests that cannabis compounds, if found to have therapeutic value, should have a good margin of safety from a neurocognitive
standpoint under the more limited conditions of exposure that would likely obtain in a medical setting.
Neuropsychological consequences of regular marijuana use: a twin study
http://journals.cambridge.org/action...ine&aid=255433
Conclusions. Out of the numerous measures that were administered, only one significant difference was noted between marijuana-using twins and their non-using co-twins on cognitive functioning. The results indicate an absence of marked long-term residual effects of marijuana use on cognitive abilities.
Chronic Cannabis Use
in the Compassionate Investigational
New Drug Program:
An Examination of Benefits
and Adverse Effects
of Legal Clinical Cannabis
http://www.medicalcannabis.com/PDF/Chronic_Cannabis.pdf

CONCLUSIONS AND RECOMMENDATIONS
1. Cannabis smoking, even of a crude, low-grade product, provides
effective symptomatic relief of pain, muscle spasms, and intraocular
pressure elevations in selected patients failing other modes
of treatment.
2. These clinical cannabis patients are able to reduce or eliminate
other prescription medicines and their accompanying side effects.
3. Clinical cannabis provides an improved quality of life in these patients.
4. The side effect profile of NIDA cannabis in chronic usage suggests
some mild pulmonary risk.
5. No malignant deterioration has been observed.
6. No consistent or attributable neuropsychological or neurological
deterioration has been observed.
7. No endocrine, hematological or immunological sequelae have
been observed.
Russo et al.
8. Improvements in a clinical cannabis program would include a
ready and consistent supply of sterilized, potent, organically grown
unfertilized female flowering top material, thoroughly cleaned of
extraneous inert fibrous matter.
9. It is the authorsā?? opinion that the Compassionate IND program
should be reopened and extended to other patients in need of clinical
cannabis.
10. Failing that, local, state and federal laws might be amended to
provide regulated and monitored clinical cannabis to suitable candidates.