A single cannabis joint could damage the lungs as much as smoking up to five tobacco cigarettes one after another, scientists in New Zealand have said.
The impact of cannabis is worsened by how joints are smoked
The research, published in the journal Thorax, found cannabis damaged the large airways in the lungs causing symptoms such as coughing and wheezing.
It also damaged the ability of the lungs to get oxygen to, and remove waste products from tissues.
Experts say the study confirms that the drug represents a serious health risk.
In the study researchers from the Medical Research Institute of New Zealand, Wakefield Hospital and the Wellington School of Medicine and Health Sciences, studied 339 volunteers.
They took CT scans of their lungs and tested their lung function through breathing tests to assess their lung damage.
Participants were divided into four groups - cannabis smokers, combined cannabis and tobacco smokers, tobacco smokers, and non-smokers, and gave them a questionnaire on their smoking habits.
Cannabis smokers were included if they had smoked at least one joint per day for at least five years, while tobacco smokers had to have smoked 20 cigarettes per day for one year.
One joint could cause as much damage as up to five cigarettes
Cannabis smokers reported symptoms such as wheezing, coughing, chest tightness and excessive phlegm production.
The drug also reduced the numbers of small, fine airways that transport oxygen and waste products to and from blood vessels in the lungs.
And it damaged the function of the large airways of the lungs, obstructing air flow and forcing the lungs to work harder, so contributing to symptoms such as coughing, and the development of bronchitis.
The extent of this large airway damage was directly related to the number of joints smoked - the more joints smoked, the more damage was seen.
However, in this study, people who smoked only cannabis were not found to suffer from emphysema, a serious and crippling lung disease which was previously thought to be linked to the drug.
The authors said: "The most important finding was that one joint of cannabis was similar to 2.5 to five tobacco cigarettes in terms of causing airflow obstruction.
They said the impact of cannabis was likely to be due to the way in which cannabis joints are smoked - joints do not usually have filters, and they reach higher temperatures with users inhaling more deeply and holding their breath for longer than cigarette smokers.
The British Lung Foundation welcomed the research, and Dr Keith Prowse, chairman of the foundation said: "This research confirms that cannabis poses a serious health risk to the lungs, and smoking a joint can be more harmful to the lungs than smoking a cigarette.
"It's important to remember, though, that tobacco continues to be more harmful overall because it is typically smoked in much higher quantities than cannabis."
The warnings come after recent research suggested cannabis smokers were 40% more likely than non-users to suffer psychotic illnesses such as schizophrenia.
Danny Kushlick, of the campaign group Transform Drug Policy Foundation, said the finding that smoking cannabis did not cause emphysema suggested it was wrong to characterise the drug as more harmful than tobacco.
He said: "The relative dangers of smoking dope are far less of a public health problem than those associated with the use alcohol and tobacco."
He also stressed that making the drug illegal did nothing to reduce any dangers that might be associated with it. In fact, he argued that its illegal status led to a lack of quality control.