When you're stoned a problem can come up and it takes a while to get your stuff together Vietnam war veteran marijuana addict

But supporters say cannabis is no more harmful or less harmful than drinking alcohol or smoking tobacco.

In a laboratory in New Orleans, rhesus monkeys were forced to smoke the equivalent of two cannabis cigarettes a day for nearly a year.

Electrodes implanted in their brains measured brain activity, and the results have indicated major if not permanent damage.

Researchers claim cannabis has a lasting impact - one monkey had not been subjected to the testing for six months, but still showed 'disrupted brain wave patterns'.

Studies on humans have assessed the effects of cannabis smoking on hormonal disruption, birth defects and severe personality disorders.

BBC correspondent John Humphreys spoke to two Vietnam War veterans taking part in the study who were heavy cannabis smokers of cannabis while on active duty.

"I just can't get my thoughts together quick enough. When you're stoned a problem can come up and it takes a while to get your stuff together to be able to deal with it. I'm still the same way now," said one.

Cannabis or marijuana is commonly used in the US - an estimated one in every eight Americans - 25 million people - have smoked the drug, and two million teenagers are regular users.

The drug is illegal in the United States for possession and consumption, and conviction for an offence often leads to a prison sentence.

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