First, it helps to understand why MJ was made illegal in the first place. For that you can thank an FBI man named Anslinger who hated black and brown people (the primary users of cannabis at the time) and he was looking for a way to lock them up, so he lobbied to get the stuff made illegal (helped out by a Hollyweird propaganda film called Reefer Madness), and made the penalties for possession as harsh as he could possibly make them.
The government also put cannabis on the Schedule I class of drugs, and those are the kind of drugs that are highly addictive and have no medicinal value whatsoever. However, since the research was started (by the governmnet btw) in the 60's to see it's effects, there is zero evidence that it is physically addictive, as well as the FACT that it has actual medicinal value.
Want to know how they could make it legal tomorrow without having to do anything? Move it to Schedule II or lower class, because that way doctors would be able to prescribe it nationwide.
As far as lung cancer? There has been research by both Harvard Medical and Britian's Royal Medical Society that shows people who smoke cigarettes only are 21 times more likely to develop lung cancer than people who smoke nothing at all. People who smoke only cannabis? They are 0.93 to 0.74 percent likely to develop lung cancer when compared to people who don't smoke anything at all.
Me personally? I say legalize it, but put the rules for it just like what they are for alcohol, 21 and over only. Why? Because the human brain isn't fully developed until a person is around 18 to 20 years old.
Anslinger was married to the niece of Treasury Secretary Andrew Mellon of Mellon Bank fame, who was a major backer of DuPont Chemical. DuPont had just synthesized a new fiber from petroleum, called nylon, and hemp fiber was it's biggest competitor.
William Randolf Hearst, the newspaper magnate, was the owner of vast tracts of timber for the pulp mills he owned that made his newsprint. (His paper was so inferior it gave rise to the term 'yellow journalism'.) Hemp was also a major competitor in the manufacturing of paper, and Hearst didn't like it. He also didn't like brown people much since Pancho Villa 'nationalized' 800,000 acres of prime Mexican timber from him. Hearst was the source of the movie 'Reefer Madness'.
IOW, marijuana faced the perfect storm of cut-throat businessmen, corrupt bankers in positions of power and virulent racism.
Needless to say, marijuana lost, and so did we.
The Marihuana Tax Act 1937