The primary case made against marijuana by the federal government was that its use had become “a national menace” in the words of Harry Anslinger, the commissioner of the Federal Bureau of Narcotics (the DEA’s predecessor). In congressional testimony Anslinger urged members to vote for the Marihuana Tax Act and described what he believed to be marijuana’s effect:

“Some individuals have a complete loss of sense of time or a sense of value. They lose their sense of place. They have an increased feeling of physical strength and power. Some people will fly into a delirious rage, and they are temporarily irresponsible and may commit violent crimes. Other people will laugh uncontrollably. It is impossible to say what the effect will be on any individual.”

Aside from raising concerns about the effects of marijuana, Anslinger stoked cultural fears by citing several instances of minorities using the drug before committing crimes, leading some to allege that racial bias played a role in its prohibition. Increasing numbers of Mexicans were immigrating to the U.S., and the use of marijuana was common among those who worked on farms in the southwest and western parts of the country. Additionally, a concentrated type of marijuana known as hashish was used by some immigrants from the Middle East and surrounding regions.

Anslinger’s congressional hearings lasted a total of one hour, while the American Medical Association testified that there is “no evidence” of marijuana’s danger. Lawmakers were unmoved. The House Ways and Means Committee held one hearing of its own before approving the bill, which was then passed by the House of Representatives as a whole following 90 seconds of debate. The Senate followed suit after holding a brief hearing shortly thereafter, sending the bill to FDR’s desk for a signature.

Here's the page it came from. Https://www.countable.us/articles/849-date-fdr-made-marijuana-illegal-81-years-ago.amp

Another article shares the same sentiment concerning Mexican immigrants.

http://www.drugpolicy.org/blog/how-did-marijuana-become-illegal-first-place