The science of studying pot always seems to be controversial. Defining "harmful" in this case as short-term harm seems to miss some of the point of the cancer finish-line at the end of Tobacco Road. Yet it does bear mentioning that marijuana smoke is harmful, as many people I've met claim otherwise even while coughing their lungs out.
Cannabis smoke is well known to be more harmful to the lungs than tobacco, because smokers inhale cannabis one-third more deeply and hold it in their lungs for up to four times as long as cigarette smokers do. Yet while we have a list of more than 4000 chemicals and toxins present in tobacco smoke from dozens of studies, there is no comparable list for cannabis.
So David Moir of Health Canada and his colleagues set out to make a direct comparison, using machines that "smoke" the cigarettes and then collect and analyse the smoke. They found that directly inhaled cannabis smoke contained 20 times as much ammonia and five times as much hydrogen cyanide as tobacco smoke.