A single cannabis joint causes the same damage to the lungs as smoking up to five cigarettes, a study published this morning has found.
Researchers from the Medical Research Institute of New Zealand examined 340 adults aged 18-70 to assess the effects of cannabis on respiratory function.
They found the effect on the lungs of each joint was equivalent to smoking between 2.5 and five cigarettes in one go.
Prof Richard Beasley and his colleagues divided the participants into four smoking categories: cannabis only; tobacco only; combined cannabis and tobacco; and non-smokers. All participants underwent high definition CT scans and took special breathing tests in order to assess how well their lungs worked.
The study, published in the medical journal, Thorax, found cannabis smoking was associated with airflow restriction and hyperinflation of the lungs. The degree of damage was related to the amount of cannabis used.
But emphysema, the crippling and progressive lung disease, was found only in those who smoked tobacco, either alone or in combination.
However, cannabis damaged the lungs by reducing the number of small fine airways; these play an important role in transporting oxygen and waste products to and from the blood.
Wheeze, chest tightness, cough and increased production of sputum (phlegm) were all associated with cannabis use.
Commenting on the finding that one joint of cannabis was similar to 2.5 to five tobacco cigarettes in terms of lung damage, the authors say this is because of the way a cannabis joint is smoked. "Cannabis is usually smoked without a filter, and to a shorter butt length and the smoke is at a higher temperature. Furthermore, cannabis smokers inhale more deeply and hold their breath for longer.
"The 1:2.5 to five dose equivalent between cannabis joints and tobacco cigarettes for adverse effects on lung function is of major public health significance," they conclude.
Meanwhile, a comprehensive analysis of the relationship between cannabis use and the development of psychosis in later life has been published in the Lancet. It concludes that the risk of psychosis increased by some 40 per cent in people who have used cannabis, with the risk rising the more the drug is used.
The risk of a psychotic episode in the most frequent users may be increased by up to 200 per cent.
The authors from Cardiff University and the University of Bristol estimated that some 14 per cent of cases of schizophrenia in young adults could be prevented if cannabis was not available.
Cannabis is used by an estimated 160 million people worldwide. In the past it had been considered a less harmful drug than heroin. However, the authors of the Lancet paper say: "We believe that there is now enough evidence to inform people that using cannabis could increase their risk of developing a psychotic illness later in life."