LONDON Smoking one cannabis joint is as harmful to a person's lungs as having up to five cigarettes, according to research published on Tuesday.
Those who smoked cannabis damaged both the lungs' small fine airways, used for transporting oxygen, and the large airways, which blocked air flow, the researchers said.
It meant cannabis smokers complained of wheezing, coughing, and chest tightness, the study by experts at the Medical Research Institute of New Zealand found.
The researchers tested 339 people -- those who smoked only cannabis, those who smoked tobacco, those who smoked both and non-smokers.
The study found only those who smoked tobacco suffered from the crippling lung disease emphysema, but cannabis use stopped the lungs working properly.
"The extent of this damage was directly related to the number of joints smoked, with higher consumption linked to greater incapacity," said the authors of the report published in the medical journal Thorax.
"The effect on the lungs of each joint was equivalent to smoking between 2.5 and five cigarettes in one go."
The British government is considering whether cannabis should be reclassified as a more serious drug because of the dangers associated with stronger strains.
"The danger cannabis poses to respiratory health is consistently being overlooked," said Helena Shovelton, Chief Executive of the British Lung Foundation.
"Smoking a joint is more harmful to the lungs than smoking a cigarette and we have just banned people from doing that in public places because of the health risks."
Last week British researchers said using marijuana increased the risk of developing a psychotic illness such as schizophrenia.