Well if you are talking about individuals - like how dangerous it is for me or you, and not talking about society then I would say the following.
The idea that cannabis is harmless, I think is really not true.
Studies have picked up the following:
*Heavy cannabis smoking can cause gum disease. A study found that after adjusting for tobacco use heavy cannabis smokers had three times the risk of having established gum disease by the age of 32. Researchers concluded this was likely to be a result of increased toxins in cannabis smoke and deeper inhalation compared to cigarettes. (Cannabis Smoking and Periodontal Disease Among Young Adults, Journal of the American Medical Association 2008 W. Murray Thomson and others).
*"Heavy cannabis users may be at greater risk of chronic lung disease - including cancer - compared to tobacco smokers." link removed "One study found a higher risk of lung cancer for those who smoked one joint a day compared with those who smoked 20 cigarettes a day over the same period. Another found bullous disease - a form of emphysema - occurs 20 years earlier in cannabis smokers."
*There may be a link between cannabis use and mental illness. Some research has suggested that cannabis users are 40% more likely than non-users to suffer a psychotic illness such as schizophrenia. Other studies suggest cannabis could be a factor in 14% of psychotic problems among young adults in the UK. (Dr Stanley Zammit, and others The Lancet 2007). However the authors note that that people who are at a higher risk of mental illness may be more likely to use the drug.
With alcohol, yes it can be very dangerous, but the difference on the individual is that if you treat it with respect (which most people do not) - for example no drink driving, sticking to government recommended limits on alcohol (admittedly who does that??) then I would say personally that I feel alcohol is safer (subject to the above conditions). However because people have to drink to excess it has the potential to be extremely dangerous.
thereforeeee I say "responsible" use of alcohol (no drink driving and remaining under government daily and weekly limits always) is (in my opinion subject to conditions) safer than cannabis. But badly used alcohol is very dangerous. I think the problem is that alcohol and it's effects are well known whereas some effects of cannabis are not yet known. Please remember that I am not medically qualified - it's just my opinion from what I have read.