Actually, in America nationwide drug laws began in the 20th century, and it had nothing to do with health or unknown effects. Opium was outlawed as part of a general effort to "protect America" from Chinese immigrants. Heroin was outlawed because a German company held a patent on it. Cocaine was outlawed because, according to the people in Congress at the time, it drove black men crazy and made it impossible to kill them with standard police-issue sidearms (also, according to an article from the New York Times just before cocaine was banned, Jews were selling it). Marijuana prohibition was principally driven by racism and as a form of regulatory capture by industries that competed with hemp.
LSD was discovered decades before it was outlawed and the effects were well understood before it was banned. LSD happens to be one of the safest recreational drugs; in fact, the biggest danger is the poorly regulated supply chain i.e. most "LSD" on the market today is not actually LSD (as with most drugs, the ban poses a greater health risk than the drug itself). The government conducted its own extremely unethical LSD study to determine if it could be used as a mind control drug (it cannot) and knew the safety profile long before the ban. LSD was banned because of concerns that it was contributing to the moral corruption of American youth (i.e. think of all those white teenagers out partying with hippies); in other words, it was viewed as a threat to the prevailing social order (probably not a coincidence that this coincided with the civil rights movement, which was a direct challenge to that social order).
Purely anecdotal, but have you ever met someone that has taken large doses of LSD over the course of a few years? They are almost always permanently changed
I also agree that the U.S. is much less of a monoculture now and people generally agree that others should be able to live, think, and believe whatever and however they want.
What do you mean by "valid" here? Do you mean they are afraid for society, or themselves?
The benign physical effects of plants like cannabis were well established by the government's own studies. The science was purposefully ignored because it did not align with the political goals of the Nixon administration.
"You understand what I'm saying? We knew we couldn't make it illegal to be either against the war or black, but by getting the public to associate the hippies with marijuana and blacks with heroin. And then criminalizing both heavily, we could disrupt those communities," Ehrlichman said. "We could arrest their leaders. raid their homes, break up their meetings, and vilify them night after night on the evening news. Did we know we were lying about the drugs? Of course we did."
http://edition.cnn.com/2016/03/23/politics/john-ehrlichman-r...