WHERE THERE'S SMOKE ...

Cannabis makes you mellow, laid-back, carefree. That is the message
from the thousands of users in New Zealand as well as lobby groups
calling for the decriminalisation of the drug.

But evidence from long-term studies around the world shows a clear
link between heavy and prolonged marijuana use and psychoses - serious
mental disorders in which people lose touch with reality - such as
schizophrenia and manic depression.

Public attention is focusing on the role of cannabis following the
horrific slaying of Dunedin meat worker Kelvin Mercer's six-month-old
baby boy and estranged wife.

The night before the murderous attack, which also saw Mercer slash the
throats of his two older daughters, the 32-year-old had sought help at
Dunedin Hospital for his heavy cannabis addiction. Mercer, who set
himself alight after the attack, died in hospital before anyone could
ask him why he did what he did.

Police say it will be weeks before test results will be able to tell
them to what extent, if at all, Mercer was under the influence of
cannabis at the time of last Thursday morning's attack. But the
question of whether cannabis played a role in turning the once caring
father into a murderer is an important one for a country where the
pro-cannabis lobby has an increasingly strong voice.

The warnings from health professionals over the health effects of the
drug are mounting.

In 1997, aged 42, David Taylor snapped. Drunk and stoned, he bashed
Maria Copley to death in Wellington's Basin Reserve.

Taylor had grown up in a dysfunctional family of seven children before
being thrown onto the streets by his violent father at the age of 14.
He spent three months living under bushes in a nearby reserve before
it sank in that he was on his own. "So I got into everything - drugs,
alcohol, crime ... it was survival. For years I could always hear the
old man telling me to go ...'You're not my son, you're not part of
this family any more'."

Describing himself as "an outcast, a loner", Taylor said he used
cannabis as an anti-depressant. "It slows things up, lets you forget
for a while."

Wellington clinical psychologist Mariette Hopman said at the time that
one of the worst things Taylor could have done was smoke cannabis.
"Smoking cannabis during adolescence is unbelievably dangerous. The
adolescent psyche is not equipped to handle it.

"There are times when I feel frantic about this and try desperately to
explain to parents that they must get this through to their kids."

She said if a young person, especially a teenager on the loose with
problems, started to smoke cannabis regularly, the long-term
repercussions could be disastrous. "It is the proverbial time bomb,
the lethal cocktail."

It has been known for 150 years that very heavy consumption of
cannabis can cause hallucinations and delusions, Professor Robin
Murray, of the London Institute of Psychiatry, told the Guardian
newspaper in Britain last year.

"This was thought to be very rare and transient until the 1980s, when
as cannabis consumption rose across Europe and the US, it became
apparent that people with chronic psychotic illnesses were more likely
to be regular daily consumers of cannabis than the general
population."

The hallucinations and delusions of schizophrenia result from an
excess of the brain chemical dopamine. Cannabis, like amphetamine and
cocaine, increases the release of dopamine in the brain.

Murray said starting early clearly increased the risk from cannabis.
"If you're taking cannabis daily by age 18, then you are about seven
times more likely to develop schizophrenia than the rest of the
population."

New Zealand should be gravely worried.

Our adolescents appear more likely to be smoking cannabis than those
in other western nations, says Professor David Fergusson, executive
director of the Christchurch Medical School's Health and Development
Study.

This study followed 1265 children born in 1977 and found more than
three-quarters had used cannabis at least once by the time they were
25. This compared with rates closer to 50 or 60 per cent in the United
States.

Fergusson's researchers also studied the incidence of psychotic
symptoms - as opposed to full-blown psychosis - such as hearing things
that are not there and delusional beliefs. After taking into account
pre-existing levels of symptoms, Fergusson said those who were heavy
cannabis users reported two times the rate of pyschotic symptoms.

His New Zealand study chimes with international evidence:

A Swedish study followed 50,000 Swedish conscripts for 15 years from
the age of 18. Those who had tried cannabis by 18 were 2.4 times as
likely to be diagnosed with schizophrenia than those who had never
used it. The risk increased the more cannabis was used.

Australian scientists reported in the British Medical Journal that
girls who smoked cannabis were five times more likely to suffer from
depression. Their findings were based on a study of 1600 teenagers
over seven years.

A British Medical Journal editorial in 2002 concluded that the link
between cannabis and psychosis was "well established". "The
explanation most accepted is that cannabis triggers the onset or
relapse of schizophrenia in predisposed people and also exacerbates
the symptoms generally ... Length of exposure to use of cannabis
predicted the severity of the psychosis, which likewise was not
explained by use of other drugs."

The evidence in relation to depression is growing. A 15-year follow-up
of an adult community sample of 1920 participants in the United States
showed that the use of cannabis increased the risk of major depression
fourfold.

Some have argued that people already suffering psychotic symptoms turn
to cannabis to find relief from their distress, that the illness
causes the cannabis addiction rather than the other way round. But
Fergusson said the "chicken and egg" argument was becoming less plausible.

"Studies all around the world suggest that heavy, serious use of
cannabis may in fact introduce pyschotic behaviours. The nature of
that link is still not known, but it does look as though heavy use may
disorder brain chemistry."

Of course, the case should not be overstated. Alcohol abuse can also
cause psychosis and psychosis occurs in only a minority of people. As
Fergusson points out, doubling a small risk still means only a small
minority of the population are likely to be susceptible. If cannabis
consumption increases - and this week the Drug Policy Forum Trust
meets in Wellington to discuss legalising it for medicinal use -
caution must be taken.