I don't think anyone here - including Capt'n Heroin - believes that cannabis is completely benign.
I understand, though, that many people tend to downplay the risks and health problems associated with cannabis. This is likely because they were taught many lies about drugs, particularly cannabis, via the government propaganda programs, and once they discovered that it was full of lies, they discard any worries about the drug at all. A potentially dangerous ignorance, I would say.
However, cannabis is clearly far less dangerous than tobacco or alcohol. It would be unfair and alarmist to put warnings on cannabis that make it appear to be more dangerous than alcohol.
One solution: put bigger warnings on alcohol. I am not opposed to this, but it brings up the question of how dangerous cannabis is compared to other drugs, such as caffeine.
I have argued elsewhere that caffeine is at least as dangerous as cannabis. The LD50 of caffeine is easily attainable by someone who is snorting pure powder (and people die from caffeine overdose in this way regularly, though infrequently).
But there are no warnings on caffeine, and to put them on coffee and tea seems impractical and ridiculous. Pure caffeine crystals, on the other hand, should have warnings, imo.
Putting them on cannabis seems like it could be beneficial, but it would be important not to use that as a chance to scare people, which many people in power seem intent on doing. Also, the scientific evidence is still quite lacking, so it is not 100% certain what would need to be conveyed. From what I know (and without looking up anything now), I would say that the basic and essential information would or could be
1) Do no drive or operate heavy machinery when under the influence.
2) Individuals with pre-existing psychosis may find that cannabis increases / brings out their psychosis.
You might think that (1) is debatable, since some evidence says that people drive better when high. Mostly, however, the
evidence says that people don't drive (much) worse when high. It really depends on exactly HOW high the person is, and what they are used to, etc., as well as the individual in question. While nobody drives better drunk, there are many people that drive just fine while high, or they are only slightly negatively affected and make up for it by driving more slowly. Overall, since the message is going out to the general public, and some people will get really high and drive and be out of it and crash and kill others, I would vote for (1).
(2) seems important for people to know.
There is absolutely NO evidence whatsoever, to my knowledge, that cannabis has ever been shown to cause cancer (despite what one poster seemed to say above). Cannabis smoke, on the other hand, may cause cancer, but so may the smoke of cloves, mint, or calea zacatekichi (sp? the "dream herb"), but we don't label these things as carcinogenic. Cannabis consumed via vaporizer or eating/drinking is not a carcinogen. If anything, it is anti-cancer - there is evidence that it causes cancer cells to die. Shouldn't that be mentioned on the package as well?