The American Medical Association (AMA) opposed the act because the tax was imposed on physicians prescribing cannabis, retail pharmacists selling cannabis, and medical cannabis cultivation/manufacturing. The AMA proposed that cannabis instead be added to the Harrison Narcotics Tax Act. The bill was passed over the last-minute objections of the American Medical Association. Dr. William Woodward, legislative counsel for the AMA objected to the bill on the grounds that the bill had been prepared in secret without giving proper time to prepare their opposition to the bill. He doubted their claims about marijuana addiction, violence, and overdosage; he further asserted that because the word Marijuana was largely unknown at the time, the medical profession did not realize they were losing cannabis. "Marijuana is not the correct term... Yet the burden of this bill is placed heavily on the doctors and pharmacists of this country."

The bill was passed on the grounds of different reports[16] and hearings.[17] Anslinger also referred to the International Opium Convention that from 1928 included cannabis as a drug not a medicine, and that all states had some kind of laws against improper use of cannabis (for ex. the Uniform State Narcotic Act). Today, it is generally accepted that the hearings included incorrect, excessive or unfounded arguments.[18] By 1951, however, new justifications had emerged, and the Boggs Act that superseded the Marijuana Tax Act of 1937 was passed.[citation needed] In August 1954, the Internal Revenue Code of 1954 was enacted, and the Marihuana Tax Act was included in Subchapter A of Chapter 39 of the 1954 Code.