Studies have shown the long-term use of cannabis to be safe.
In contrast to many other medicinal drugs, the long-term use
of cannabis does not harm stomach, liver, kidneys and heart.
The Missoula Chronic Clinical Cannabis Use Study examined
the effects of long-term and legal medical marijuana use.
Russo et al. (2002) demonstrated that regular use of cannabis
for more than ten years does not cause major harm to patients:
"The Missoula Chronic Clinical Cannabis Use Study was
proposed to investigate the therapeutic benefits and adverse
effects of prolonged use of "medical marijuana”
in a cohort of seriously ill patients. Use of cannabis was
approved through the Compassionate Investigational New Drug
Program (IND) of the Food and Drug Administration (FDA). Cannabis
is obtained from the National Institute on Drug Abuse (NIDA),
and is utilized under the supervision of a study physician.
The aim of this study is to examine the overall health status
of 4 of the 7 surviving patients in the program. This project
provides the first opportunity to scrutinize the long-term
effects of cannabis on patients who have used a known dosage
of a standardized, heat-sterilized quality-controlled supply
of low-grade marijuana for 11 to 27 years. (...)
Results demonstrate clinical effectiveness in these patients
in treating glaucoma, chronic musculoskeletal pain, spasm
and nausea, and spasticity of multiple sclerosis. All 4 patients
are stable with respect to their chronic conditions, and are
taking many fewer standard pharmaceuticals than previously.
Mild changes in pulmonary function were observed in 2 patients,
while no functionally significant attributable sequelae were
noted in any other physiological system examined in the study,
which included: MRI scans of the brain, pulmonary function
tests, chest X-ray, neuropsychological tests, hormone and
immunological assays, electroencephalography, P300 testing,
history, and neurological clinical examination. (...)
These results would support the provision of clinical cannabis
to a greater number of patients in need. We believe that cannabis
can be a safe and effective medicine with various suggested
improvements in the existing Compassionate IND program”
(Russo et al. 2002).
The Missoula Chronic Clinical Cannabis Use Study resulted
in several important conclusions and recommendations:
- "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.“
- "These clinical cannabis patients are able to reduce
or eliminate other prescription medicines and their accompanying
- "Clinical cannabis provides an improved quality of
life in these patients.“
- "The side effect profile of NIDA cannabis in chronic
usage suggests some mild pulmonary risk.“
- "No malignant deterioration has been observed.
- "No consistent or attributable neuropsychological
or neurological deterioration has been observed.“
- "No endocrine, hematological or immunological sequelae
have been observed.“
- "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.“
- "It is the authors’ opinion that the Compassionate
IND should be reopened and extended to other patients in
need of clinical cannabis.“
- "Failing that, local, state and federal laws might
be amended to provide regulated and monitored clinical cannabis
to suitable candidates“ (Russo et al. 2002).
Research on prenatal marijuana exposure found that cognitive
functions of children at school age may be impaired. However,
these effects seem to be mild and were considerable less compared
to alcohol and tobacco. A report from a longitudinal study
of the effects of prenatal alcohol and marijuana exposure
investigated whether these drugs affect neuropsychological
development at 10 years of age (Richardson et al. 2002). 593
children completed a neuropsychological battery. Prenatal
alcohol use was found to have a significant negative impact
on learning and memory skills. Prenatal marijuana exposure
also had a minor effect on learning and memory. Another study
assessed cognitive performance in new-borns of 354 mothers
at age 6.5, 12, and 13 months (Jacobson et al. 2002). Alcohol
use during pregnancy was associated with poorer cognitive
performance. The use of cocaine and tobacco was associated
with a smaller size at birth. No effects were detected in
relation to cannabis use. Low density of cannabinoid receptors
in the fetal brain may explain the low prenatal toxicity of
cannabis (Biegon and Kerman 2001). Researchers found that
low numbers of cannabinoid receptors could be observed as
early as the 14th week of gestation. Receptor density increased
slowly but did not reach adult levels by the end of the 24th
week. The distribution pattern in the fetal brains was markedly
different from the adult pattern. Authors conclude:
"The relatively low and regionally selective appearance
of cannabinoid receptors in the fetal human brain may explain
the relatively mild and selective nature of postnatal neurobehavioral
deficits observed in infants exposed to cannabinoids in utero"
(Biegon and Kerman 2001).
The long-term consequences on cognitive function are also
a major topic of discussion with regard to adult cannabis
use. The first longitudinal study examining the development
of cogntive functioning conducted in the U.S. did not find
any influence of cannabis use (Lyketsos et al. 1999). This
was confirmed by a later Canadian study (Fried et al. 2002)
According to the large-scale study by Lyketsos et al. (1999),
the age-related decline of cognitive functioning "...does
not appear to be associated with cannabis use." Constantine
Lyketsos and colleagues of John Hopkins Hospital in Baltimore
conducted a follow-up study of 1,318 people, divided into
heavy users, light users, and nonusers of cannabis. All participants
had completed a special test, the Mini Mental State Examination
(MMSE), in 1981, 1982, and 1993-1996. The individual score
differences between 1982 and 1993-1996 were calculated for
each study participant. Within these 12 years, the mean score
decline for all groups was 1.2 points. The Mini Mental State
Examination (MMSE) is a brief and widely used standardized
method for assessing cognitive mental status. It assesses
orientation, attention, immediate and short-term recall, language,
and the ability to follow simple verbal and written commands.
The maximum achievable score is 30. Researchers found a decline
in all age groups. There was "no significant differences
in cognitive decline between heavy users, light users, and
nonusers of cannabis." There were also no differences
attributable to sex in these subgroups.
Former studies have been hampered by the fact that they are
based on retrospective studies with single measurements. In
a commentary by Martha Clare Morris and colleagues of the
Rush Institute for Healthy Aging in Chicago, the difficulties
encountered with the use of single measurements of cognition
and the importance of measuring changes are stressed (Morris
et al. 1999).
In the second longitudinal study ever conducted, Canadian
researchers did not find any long-term effect of heavy cannabis
use on overall intelligence (Fried et al. 2002). They compared
the intelligence quotient (IQ) of 15 current heavy users of
cannabis, 9 current light users, 9 former regular users and
37 non-users in a group of 70 young people. Participants had
been followed since birth and now were 17-20 years of age.
Current marijuana use was significantly correlated in a dose-related
fashion with a decline in IQ when compared to the IQ measured
at age 9-12. In current heavy users, the IQ showed a decrease
of 4.1 points, compared to gains in IQ points for light current
users (5.8), former users (3.5) and non-users (2.6). The authors
concluded that current cannabis use "had a negative effect
on global IQ score only in subjects who smoked 5 or more joints
per week" and that "marijuana does not have a long-term
negative impact on global intelligence."
U.S. research at Harvard Medical School showed that cognitive
impairment after regular heavy use is reversible (Pope et
al. 2001). Three groups of individuals aged 30 to 55 years
were compared with regard to their cognitive abilities: (1)
63 current heavy users who had smoked cannabis at least 5000
times in their lives and who were smoking daily at study entry;
(2) 45 former heavy users who had also smoked at least 5000
times but fewer than 12 times in the last 3 months; and (3)
72 control subjects who had smoked no more than 50 times in
their lives. Results showed that some cognitive deficits appear
detectable at least 7 days after discontinuation of heavy
cannabis use. By day 28, however, there were virtually no
significant differences among the groups on any of the test
results. Authors concluded
"Some cognitive deficits appear detectable at least
7 days after heavy cannabis use but appear reversible and
related to recent cannabis exposure rather than irreversible
and related to cumulative lifetime use" (Pope et al.
However, the discussion on whether regular cannabis use causes
a decline in cognitive function continues, as can be seen
from a discussion in the Journal of the American Medical Association
in March and May 2002 (Solowij et al. 2002, Nyquist 2002,
Watson 2002, Gunderson et al. 2002, Pope 2002).
Govermental and expert committees in several industrialized
countries have also concluded that the side effects of cannabis
are relatively benign, supporting its safety even for prolonged
use. The Institute of Medicine Report of 1999 states:
"Marijuana is not a completely benign substance. It
is a powerful drug with a variety of effects. However, except
for the harms associated with smoking, the adverse effects
of marijuana use are within the range of effects tolerated
for other medications. (…)
The Canadian Senate’s Special Committee on Illegal
Drugs has studied the effects of cannabis use for 14 months.
The committee states in a premilinary report issued in May
2002 that scientific evidence suggests that marijuana "may
have some negative effects on the health of individuals,"
but that these effects would be "relatively benign"
and that marijuana is no gateway drug to the use of hard drugs.
Only approximately 10 percent of the users would become chronic
users and 5 to 10 percent would become addicted. The preliminary
report is available at the web site of the parliament at
A select committee of Britain's House of Lords recommended
that cannabis should be rescheduled from a schedule 1 to a
schedule 2 drug under the Misuse of Drugs Regulations Act
of 1985, since it was not a dangerous drug and in order to
facilitate medical research with cannabis (House of Lords
1998). The committee accused the Medicines Control Agency
of not dealing with cannabis-based medicines in the same impartial
manner as with other medicines (Hause of Lords 2001). In the
second report released on 22 March 2001, the select committee
on science and technology also called for an end to the prosecution
of therapeutic cannabis users who possess or grow cannabis
for their own use.
After eight months of deliberation, a health select committee
of the parliament of New Zealand tabled its report on research
into the mental health effects of cannabis on December 17,
1998, finding that the drug has probably been unduly criticised
(New Zealand Herald from 18 December 1998). "Based on
the evidence we have heard in the course of this inquiry,"
the committee concluded, "the negative mental health
impact of cannabis appears to have been overstated, particularly
in relation to occasional adult users of the drug." "Evidence
received in the course of this inquiry has raised serious
doubts about commonly held beliefs about cannabis," wrote
the committee. "Evidence received during the inquiry
supports the view that there can be subtle cognitive impairment
in cannabis users," the report says. In this respect,
the committee drew to a large extent on the work of Prof.
Wayne Hall of the Australian National Drug and Alcohol Research
Centre, who was commissioned to report on scientific research
in this area. He found that long-term use of cannabis may
cause subtle impairment in the higher cognitive functions
of memory, attention and the organisation and integration
of complex information. The committee said the evidence also
suggested that cannabis did not cause behavioral difficulties,
but rather that cannabis was frequently used by youths who
misbehaved; neither was it a cause of suicide.
On 22 November 2001, the French National Health and Medical
Research Institute (Inserm, Institut National de la Santé
et de la Recherche Médicale) presented a 58-page literature
review with the title "Cannabis - which effects on behaviour
and health?" (Inserm 2001). The report was ordered by
a governmental working group on the fight against drugs and
drug addiction. Main topics of the report are factors that
influence use, acute and chronic effects, and groups of special
interest (pregnant women, individuals with mental disorders).
It did not deal with the medical use of cannabis. The report
stated that about 10 percent of those who ever used cannabis
have a risk to become dependent, compared to 30 percent with
tobacco, and that cannabis effects on the nervous system are
functional and reversible, and do not cause long-term damage.
In response to these findings on the long-term safety of
cannabis use, many countries are now relaxing their cannabis
laws or are discussing legal access to medical cannabis, among
them several European countries, Australia, New Zealand, and
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