A single cannabis joint has the same effect on the lungs as smoking up to five cigarettes in one go, indicates research published ahead of print in the journal Thorax.
The researchers base their findings on 339 adults up to the age of 70, selected from an ongoing study of respiratory health, and categorised into four different groups.
These comprised those who smoked only cannabis, equivalent to at least one joint a day for five years; those who smoked tobacco only, equivalent to a pack of cigarettes a day for at least a year; those who smoked both; and those who did not smoke either cannabis or tobacco.
All the participants had high definition x-ray scans (computed tomography) taken of their lungs and they took special breathing tests designed to assess how well their lungs worked.
They were also questioned about 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.
Combined smokers tended to use less tobacco, the findings showed.
Cannabis smokers complained of wheeze, cough, chest tightness and phlegm. But emphysema, the progressive and crippling lung disease, was only seen in those who smoked tobacco, either alone or in combination.
But cannabis still damaged the lungs and stopped them from working properly.
It diminished the numbers of small fine airways, which are important for transporting oxygen and waste products to and from the blood vessels effectively.
And it damaged the large airways of the lung, blocking airflow, and forcing the lungs to work harder.
The extent of this damage was directly related to the number of joints smoked, with higher consumption linked to greater incapacity.
The effect on the lungs of each joint was equivalent to smoking between 2.5 and five cigarettes in one go.
The authors explain that the impact of cannabis is strongly associated with the way in which it is smoked. It is usually smoked without a filter, and at a higher temperature. Smokers tend to inhale more deeply and to hold their breath for longer.