One Cannabis Joint Equals Smoking Up to Five Cigarettes

WELLINGTON, New Zealand, July 31 -- For heavy-duty users of marijuana, the joint is tough on the lungs, researchers here found.

In fact, a single marijuana joint for such patients can cause more lung damage than 2.5 to five cigarettes, Richard Beasley, MBChB, FRACP, of the Medical Research Institute of New Zealand here, and colleagues, reported online in Thorax.

Cannabis, the plant source of marijuana, was associated with a dose-related impairment of the large airways, causing airflow obstruction and hyperinflation, the researchers said. But unlike cigarette smoking, cannabis was rarely associated with macroscopic emphysema,

Cannabis is the most widely used illegal drug worldwide. Long-term use is known to cause chronic bronchitis and airflow obstruction, but the prevalence of macroscopic emphysema, the dose-response, and the dose equivalence of cannabis with tobacco had not been determined, Dr. Beasley said.

A sample of 339 adults, ages 18 to 70, was recruited from the Wellington region and put into four groups: cannabis only (75 people), tobacco only (92 people), combined cannabis and tobacco (91), and non-smokers of either substance (81).

Because of the small number of individuals who smoked cannabis in the original random population sample, the investigators used a convenience sample through newspaper and radio advertisements.

Respiratory status was assessed with high-resolution CT scans, pulmonary function tests, and a respiratory and smoking questionnaire. Associations between respiratory status and cannabis use were analyzed with covariance and logistic regression.

Inclusion criteria were a lifetime exposure of at least five joint-years of cannabis or at least one pack-year of tobacco. A joint-year of cannabis was defined as smoking one joint a day for one year and a pack-year of tobacco was equivalent to smoking 20 cigarettes a day for a year.

A dose-response relationship was found between cannabis smoking and reduced forced expiratory volume (FEV₁) to forced vital capacity ratio and specific airways conductance, and increased total lung capacity.

For measures of airflow obstruction, one cannabis joint had a similar effect to 2.5 to five cigarettes, the researchers reported.

Airflow obstruction and hyperinflation are a consequence of the large airway impairment, probably due to inflammation and edema in the tracheobronchial mucosa of cannabis smokers, the researchers said.

Cannabis smoking was also associated with decreased lung density on high-resolution CT scans.

Wheezing was linked to both cannabis (OR 1.3) and tobacco smoking (OR 1.4), with no evidence of an interaction.

Chest tightness was associated with cannabis (OR 1.4) but not with tobacco smoking (OR 1.1).

Cough was associated with both cannabis (OR 1.5) and tobacco (1.9).

Chronic bronchitis was likelier with cannabis than with tobacco use (OR 2.0 versus 1.6).

The combined effects of smoking both cannabis and tobacco was to attenuate all these associations, the researchers reported.

On the other hand, macroscopic emphysema was seen in only those who smoked tobacco, either alone or in combination with cannabis.

Emphysema was detected in only one of the cannabis smokers (1.3%), in 15 (16.3%) of the cigarette smokers, in 17 (18.9%) of the combination smokers, and in none of the non-smoking groups.

This suggests that cannabis does not cause emphysema when smoked in sufficient quantities to cause airflow obstruction, hyperinflation, and chronic bronchitis, the researchers said.

The dose equivalence found in this study, the researchers said, is consistent with the reported three- to five-fold greater levels of carboxyhemoglobin and tar inhaled when smoking a cannabis joint compared with a tobacco cigarette of the same size.

This pattern is likely to relate to the different characteristics of the cannabis joint and the way it is smoked. Cannabis is usually smoked without a filter and to a shorter butt length, while the smoke temperature is higher.

Furthermore, the investigators said, cannabis smokers inhale more deeply, hold their breath longer, and perform the Valsalva maneuver at maximal breath hold.