Marijuana has very weak medicinal values that can easily be replaced by far more effective and capable forms of medicine, that also don't make you high all the time.
Marijuana has more or less been shown to lower I.Q. It permanently changes the brain by decreasing the size of the hippocampus, but that aside there's also increased chance of cancer, particularly of the lung if you smoke it. Like inhaling any smoke does.
Further, it lowers the reaction time of people, and generally removes them from the situation making them less attentive, which in turn makes them worse drivers, among other things.
As for studies that tried to show it was safer, objectively, substantially more people die from marijuana related accidents than are chronic users. The test showed people drove half of what the speed limit said to, and then concluded this made them safer. In real world conditions, driving that slow can cause an accident, especially on the highway, and the inability to stop in time by driving the actual speed of traffic means they'd be more likely to cause a car accident or hit somebody. Increased rates of heart disease are also usually present. ]
Car accidents are more common, so that's a chance of death. The chance of a THC overdose is overall pretty low, just like it is with nicotine. In fact, no person has died from a nicotine overdose in over 10 years. However, as anybody can tell you cigarettes are harmful to your health. Not dying of an overdose is not the only way to die, or the only negative impacts. Because dying of old age means heart disease or cancer, the fact it speeds both of those things up basically means they count it as shaving years off your life. As a chronic user you can generally expect to die 10-20 years earlier.
Now, if we as a society are willing to take the risks, that's a different story all together; after all, plenty of things like say, rat poison are legal. But, these are just some facts to bear in mind.
Edited by Manoka, 18 December 2015 - 12:28 AM.