Without actually doing any research, here is all I know:
Salvia Divinorum is a combination of various plants and chemicals, and is sold as an incense. Most Salvia packaging has a disclaimer that reads "company is not liable for misuse of this product". It is the same case as the other legal incenses such as LOL, K2, or King Karma. They are sold for aromatic therapeutic purposes and are very unhealthy to orally smoke. The euphoric effects are known, but not enough to justify making them illegal. However, a recent DEA federal ban that took place in March made the manufacturing and distribution of Salvia illegal, as well as many common and main ingredients in the legal "smokes" such as the many variations of the JWH compound.
As far as marijuana goes, there are many reasons why it is illegal. Obviously the impairment of a person's judgment skills. The main reason that I know of is the fact that marijuana can be grown anywhere if the right climate control is used, unlike tobacco. Therefore if it was legal the government could not tax it if it was grown on a private residence. Scientific studies have proven over and over again how much safer marijuana is than alcohol, hence why you've probably never heard the term, "medical alcohol".
Marijuana is a schedule I drug and is perceived as addictive. I personally don't understand that argument because aren't cigarettes and alcoholic beverages perceived as addictive as well?
That's all I got off the top of my head, hope this helps.
Posted 2759 day ago