Tetrahydrocannabinol(THC) is toxic. Are you aware of this.
This is the chemical defense of cannabis to prevent it from being consumed by herbivores.
There is a increased risk of heart attack with prolonged THC exposure.
Side effects observed in double blind clinic studies observed include nausea, increased weakness, behavioral or mood changes, suicidal ideation, hallucinations, dizziness, and vasovagal symptoms, fatigue, and feelings of intoxication.
Then you factor in some of the same risk of smoking marijuana and smoking tobacco in regards to your lungs.
The difference is I do not know of very many people have the tolerance, time, and money to smoke 20 joints a day unlike cigarettes.