As the British Lung Foundation warns that a "dangerous" lack of awareness about the dangers of smoking cannabis could be putting millions at risk, experts discuss the health problems of the drug - including the fact that each cannabis cigarette increases the chances of developing lung cancer as much as smoking 20 normal cigarettes.

By Lisa Salmon.

While cannabis is the most commonly used illicit drug in the UK, with more than two million people admitting to having used it in the past year, a leading charity claims there is still a dangerous lack of public awareness of how harmful it can be.

The British Lung Foundation (BLF) has just published a report which asserts that the risk of developing lung cancer is up to 20 times greater from a cannabis joint than a legal tobacco cigarette.

But the report has been condemned by both the respected expert on drugs Professor David Nutt, the Government's former chief drugs adviser, who points out that it is "unfounded inference", and cannabis law reform campaigning group CLEAR, which insists it's "scaremongering and exaggeration".

Fatal impact claims The BLF report says there are "strong associations between smoking cannabis and many lung and respiratory illnesses, including tuberculosis, acute bronchitis and lung cancer".

It says the practice is also strongly associated with suppression of the immune system and heart disease.

Yet, according to the charity, there is an "alarming disconnect" between the public perception of cannabis as a relatively safe drug and the impact it can have on the lungs of people who smoke it.

The charity states that the risk of developing lung cancer is up to 20 times greater in a cannabis cigarette than in a tobacco cigarette - yet 88% of the 1,045 British people it questioned believed tobacco cigarettes pose the greater risk.

Dame Helena Shovelton, the BLF's chief executive, said: "It is alarming that, while new research continues to reveal the multiple health consequences of smoking cannabis, there is still a dangerous lack of public awareness of quite how harmful this drug can be.

"Young people in particular are smoking cannabis unaware that, for instance, each cannabis cigarette they smoke could increase their chances of developing lung cancer by as much as an entire packet of 20 tobacco cigarettes."

The BLF says the average puff on a cannabis cigarette is two-thirds larger and is held for four times longer than the average puff on a tobacco cigarette.

As a result, it says, someone smoking a cannabis cigarette inhales much more tar and carbon monoxide.

Cannabis is one of the most widely-used recreational drugs in the UK, with almost a third of the population having tried it.

Dame Helena says: "This is not a niche problem. We need a serious public health campaign to finally dispel the myth that smoking cannabis is a safe pastime."

The counter-claims However Professor David Nutt, chairman of the Independent Scientific Committee on Drugs (ISCD), which aims to give ordinary people reliable information about the effects and risks of drugs, suggests that while the BLF is an "admirable charity", its claims about the harm cannabis may do to the lungs are unfounded.

He says that while the BLF report references a great deal of scientific evidence and acknowledges the research findings are mixed, it focused on just one study, by Aldington in 2008, and "came to firm conclusions that the most alarming interpretation of the most alarming evidence was true".

He says the report's interpretation that a cannabis joint may be as carcinogenic as up to 20 cigarettes is "dubious".

Prof Nutt stresses that the BLF's "worthy intentions to help us all look after our lungs" can't be doubted, and the report features important harm-reduction messages about cannabis smoke, specifically that if people insist on using the drug, rolling with tobacco may increase risk of harm and using a cannabis vaporiser instead may decrease harm.

But he suggests that the BLF's "lack of care with the evidence" could have the opposite effect from their good intentions, and warns: "If the BLF's misguided information is believed, people could actually be put at greater risk of lung cancer, for example by cutting down on the cannabis in their joints and padding them out with more tobacco."

Dr Tim Williams, a consultant addiction psychiatrist and member of the ISCD, suggests the BLF "got their evidence slightly mixed up", and points out: "We don't really know whether cannabis is more harmful than tobacco, so to say that 88% of the public don't know about the risks is not true."

However, Dame Helena counters: "Regardless of the dangers of smoking tobacco, which are already well-known, our report wanted to outline how smoking cannabis can also be harmful. While we clearly acknowledge areas in which the evidence is mixed, we did also consider it our duty to highlight the potential risks of smoking cannabis that were revealed in the research.

"This is especially in light of our own research revealing that a third of the population considered smoking cannabis to be not harmful to their health."

Complicated mix Dr Williams says that cannabis research is complicated by the fact that most cannabis is usually smoked with tobacco but, like the BLF, he emphasises that more research is needed.

"We need to be honest and say we don't actually know the risks associated with cannabis," he says.

What is known is that teenage cannabis smokers have a higher risk of developing psychotic symptoms later in life, perhaps because their developing brains are more vulnerable.

However, while cannabis use has gone up enormously in the last 50 years in the UK, he points out that as the levels of schizophrenia haven't. It's fair to say that there's no global effect of cannabis causing schizophrenia or psychotic illness in older brains that don't already have mental problems.

While Dr Williams disputes the BLF's conclusions about cannabis and lung cancer, he says that the drug does have a higher concentration of certain carcinogens (cancer-causing compounds) than nicotine.

However, the potential cancer risks are reduced as cannabis users generally smoke fewer cigarettes than tobacco smokers and most give up the drug in their 30s, thus limiting long-term exposure.

Symptom relief Evidence is emerging that cannabis is beneficial to people with multiple sclerosis (MS), says Dr Williams, and in 2010 the symptom relief drug Sativex, which includes cannabis extract, was licensed for the treatment of spasticity in people with MS. Trials have shown the treatment to be effective in around 50% of those who take it, says the Multiple Sclerosis Society.

It has been suggested that cannabis-based compounds also have a role in other disorders involving spasticity and pain, and that they may have a role as neuroprotective agents.

A much-debated point about cannabis is whether its use leads people to use stronger drugs such as heroin. Dr Williams says that while studies have shown that cannabis use does have some predictive value for future heroin use, "it should be noted that early alcohol use and cigarette smoking also predicts future cannabis use".

He says there are cannabinoid receptors in the brain, and points out: "It's an inherent system that's affected by cannabis, so it would make sense that there are some medicinal purposes for the drug. But we need much more evidence."

He adds: "We can be certain that there are some elements of harm to cannabis, but when put against the harm associated with alcohol, tobacco and heroin, for example, it's much lower.

"We have to accept there's a large amount of cannabis smoking out there, so what can we do to reduce the risk to these people?"

Cannabis facts :: Two million people in the UK smoke cannabis, and half of all 16 to 29-year-olds have tried it at least once.

:: Cannabis was re-classified in January 2009 and is now a Class B drug. The maximum penalty for possession is five years in prison and/or an unlimited fine, or both, and 14 years in prison and/or an unlimited fine for dealing or supplying.

:: Around one-in-10 cannabis users have unpleasant experiences, including confusion, hallucinations, anxiety and paranoia.

:: The amount of the main psychoactive ingredient, THC, in herbal cannabis varies from 1%-15%, and newer strains, including skunk, can contain up to 20%.

:: Cannabis Law Reform (CLEAR), formerly known as the Legalise Cannabis Alliance, is running a campaign to encourage those who use cannabis not to smoke it with tobacco. The Tokepure campaign points out that cannabis taken with a vaporiser "carries none of the risks of smoking".

:: The Royal College of Psychiatrists has produced an information leaflet on cannabis and mental health (available from