When was marijuana first illegal?

The first marijuana law found so far was a 1905 El Paso, Texas law. It, and most of the other laws in the southwest that followed, was motivated by racial prejudice against Mexican immigrants. These laws had nothing to do with the actual effects of the marijuana. In fact, cannabis in the form of hemp was a common crop in many of these states, and tinctures of cannabis were included in hundreds of common medicines. The purpose of the laws was to discriminate against Mexicans and other racial minorities. This same purpose is also found in the history of the laws against opiates and cocaine. The first state law was a 1913 California law that received little notice. It was promoted by the pharmaceutical industry that saw marijuana as a competitor. That was followed by a 1914 Utah level that was simply a Mormon religious prohibition enacted into law. By 1930, about thirty states had passed laws against marijuana for one reason or another. In the southwest and south, it was primarily racial prejudice against Mexicans and other racial minorities. In the northern states it was primarily the fear that heroin addiction would lead to the use of marijuana -- exactly the opposite of the modern marijuana gateway myth.

As far as the people involved, two people get primary credit. One is Harry Anslinger, who actively promoted Reefer Madness because he knew the marijuana laws were unenforceable. Therefore, he reasoned that the only possible method was to scare people so bad that they would never touch it. His plan worked for a while.

The other is Dr. James C. Munch of Temple University. There were only two doctors who testified for the congressional hearings. One was the representative of the American Medical Association. He said that there was no evidence that marijuana was a dangerous drug. The committee basically told him to shut up and leave.

The other was Dr. James C. Munch. His sole claim to fame was that he had injected some extract of cannabis directly into the brains of 300 dogs, and two of them had died. When they asked him what he concluded from that, he said he didn't know. However, he was the only doctor in the US who agreed that marijuana should be illegal, so his testimony was accepted, and he later became the US Official Expert on Marihuana. While serving in that capacity, he also testified in court, under oath, that marijuana would make your fangs grow six inches long and drip with blood and, when he tried it, it turned him into a bat.

Anslinger served as director of the Federal Bureau of Narcotics from 1930-1962. Dr. Munch served as US Official Expert on Marihuana from 1938-1962.

References:

Short History of the Marijuana Laws - http://druglibrary.org/schaffer/History/whiteb1.htm

The Forbidden Fruit and the Tree of Knowledge -- the Legal History of American Marihuana Prohibition - http://www.druglibrary.org/schaffer/Library/studies/vlr/vlrtoc.htm

Complete transcripts of the congressional hearings for the Marihuana Tax Act of 1937, and related papers -- http://druglibrary.org/schaffer/hemp/taxact/taxact.htm