Weed is no worse than tobacco. More chemicals are put into tobacco than weed, and it is also more addictive.
on the basis of the items themselves, this is entirely true. tobacco is packed with additives, to promote the smoke consumers expect, but also to maintain burn. such compounds also promote an addictive property; ie a physiological dependence, whereas cannabis is NOT at all addictive, but simply habitual (hence Perp and his nightly puffs)
however, the behaviours associated with spliff smoking can make it comparably unhealthy to cigarettes etc. you see the popularised imagery of taking a toke, breathing deeply (usually thru teeth for why?) and holding it forever, before billowing out a cloud. no cigarette smoker enjoys their drug like this!
the action of retaining smoke in the lungs promotes the risk of adopting carcinogenic properties. deepening the inhalation also ensures more smoke forms a residual volume in your lungs.
for spliffs to be less damaging than cigarettes YOU MUST:
-toke as per a regular ciggie
-do not inhale deeply, but exhale as much as you possibly can
(a good tip is also to cough like a maniac after you stub your blunt out)
-don't mix tobbacco in with the herb.
all things considered, cannabis is one of the oldest ever cultivated plants in human civilisations; it is useful for a myriad number of things; it has been widely studied the world over (altho less in the USA due to draconian laws) and of all narcs, it is the most harmless, being less damaging than booze or fags.