First, let's look at the text of the 18th and 21st Amendments:
Passed by Congress December 18, 1917. Ratified January 16, 1919. Repealed by amendment 21.
After one year from the ratification of this article the manufacture, sale, or transportation of intoxicating liquors within, the importation thereof into, or the exportation thereof from the United States and all territory subject to the jurisdiction thereof for beverage purposes is hereby prohibited.
The Congress and the several States shall have concurrent power to enforce this article by appropriate legislation.
This article shall be inoperative unless it shall have been ratified as an amendment to the Constitution by the legislatures of the several States, as provided in the Constitution, within seven years from the date of the submission hereof to the States by the Congress.
Passed by Congress February 20, 1933. Ratified December 5, 1933.
The eighteenth article of amendment to the Constitution of the United States is hereby repealed.
The transportation or importation into any State, Territory, or possession of the United States for delivery or use therein of intoxicating liquors, in violation of the laws thereof, is hereby prohibited.
This article shall be inoperative unless it shall have been ratified as an amendment to the Constitution by conventions in the several States, as provided in the Constitution, within seven years from the date of the submission hereof to the States by the Congress.
In both Amendments, the phrase "intoxicating liquors" is used. Mirriam-Webster defines a liquor as
a liquid substance: as
a : a usually distilled rather than fermented alcoholic beverage
b : a watery solution of a drug
The Oxford English Dictionary gives a similar definition, including:
- A liquid produced or used in a process, in particular:
2.1 Liquid in which something has been steeped or cooked.
2.2 Liquid which drains from food during cooking.
2.3 The liquid from which a substance has been crystallized or extracted.
Based entirely on those, one could argue that liquid versions of some drugs could be considered liquors, if you were to modify them a little. However, this is not necessarily the case in the eyes of the government and related agencies. The CDC says
Ethyl alcohol, or ethanol, is an intoxicating ingredient found in beer, wine, and liquor. Alcohol is produced by the fermentation of yeast, sugars, and starches.
This seems to imply that alcohol must be present in liquors. Also, other laws use the phrase "intoxicating liquor" to describe a class of alcoholic beverages; see, for example, Section 311.020 of Chapter 311, Liquor Control Law, of Missouri.
Furthermore, given that the term "liquor" is often used colloquially to refer to malt liquors, it could be argued that a reasonable person would understand that liquors contain alcohol.
The above arguments rule out the possibility of other drugs being considered by the 18th and, thus, 21st Amendments.
The reasoning of nomen agentis provides an explanation for why the specific argument you give is invalid. It boils down to "All alcoholic beverages contain drugs, but not all drugs contain alcohol."
The reasoning you give is invalid.