MARIJUANA LINKS TO CANCER GOING UP IN SMOKE

Although marijuana and tobacco smoke are packed with cancer-causing
chemicals, other qualities seem to keep it from promoting lung cancer,
an American report says.

The difference rests in the often opposing actions of the nicotine in
tobacco and the active ingredient, THC, in marijuana, said Robert
Melamede, of the University of Colorado, who reviewed the scientific
evidence supporting this contention in a recent issue of the Harm
Reduction Journal.

Whereas nicotine has several effects that promote lung and other types
of cancer, THC acts to counter the cancer-causing chemicals in
marijuana smoke - it "turns down the carcinogenic potential", Dr
Melamede said.

Laboratory research indicates that nicotine activates a body enzyme
that converts certain chemicals in tobacco and marijuana smoke into
cancer-promoting form. In contrast, studies in mice suggest that THC
blocks this enzyme activity.

Another key difference, he said, is in the immune-system effects of
tobacco and marijuana. Smoke sends irritants into the respiratory
system that trigger an immune-regulated inflammatory response, which
involves the generation of potentially cell-damaging substances called
free radicals. These are thought to contribute to several diseases,
including cancer.

But cannabinoids - both those found in marijuana and the versions
found naturally in the body - have been shown to reduce this
inflammatory response, Dr Melamede said.

Another difference between tobacco and marijuana smoking involves
cells that line the respiratory tract.

While these cells have receptors that act as docks for nicotine,
similar receptors for THC and other cannabinoids have not been found.

Dr Melamede said nicotine appears to keep these cells from committing
"suicide" when they are genetically damaged, by smoking, for instance.
When such cells do not kill themselves off, they can become tumours.
THC, however, does not appear to act this way in the respiratory tract.

All of this backs studies that have failed to link marijuana smoking
with a higher risk of lung cancer, although there is evidence that pot
users have more problems such as chronic cough, and frequent
respiratory infections, he said.

If marijuana does not promote lung cancer, that could be a factor in
the debate over the medical use of marijuana. Dr Melamede believes
"marijuana has loads of medicinal value", from conditions such as
multiple sclerosis, to chronic arthritis pain, to nausea caused by
cancer treatment.