The latest research says the effects of a single cannabis 'joint' could damage the lungs as much as smoking up to five tobacco cigarettes one after another.
The scientists say the effects of cannabis is made worse by how the joints are smoked and that the drug is capable of damaging the large airways in the lungs causing symptoms such as coughing and wheezing.
The New Zealand scientists say cannabis also damages the ability of the lungs to get oxygen and remove waste products from tissues.
Experts say the research confirms suspicions that the drug represents a serious health risk to the lungs, and that smoking a joint can be more harmful to the lungs than smoking a cigarette.
For 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 up to the age of 70.
By taking CT scans of their lungs and testing their lung function through breathing tests they were able to assess their lung damage.
The 339 participants were divided into four groups: - cannabis smokers, combined cannabis and tobacco smokers, tobacco smokers, and non-smokers.
All were given a questionnaire on their smoking habits.
Seventy five people smoked only cannabis, and 91 smoked both; eighty one people did not smoke either, and 92 smoked only tobacco.
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.
The tests revealed that one joint could cause as much damage as up to five cigarettes and also reduced the numbers of small, fine airways that transport oxygen and waste products to and from blood vessels in the lungs.
Cannabis smokers reported symptoms such as wheezing, coughing, chest tightness and excessive phlegm production.