Is prohibition unconstitutional?
When it became law, 1920, it was not.
Prohibition in the United States lasted from 1920 to 1934. It was legal during that period only because an Amendment to the Constitution had been passed by the required 2/3 of the states. Such law cannot be challenged in the courts, not even the Supreme Judicial Court. This is because the courts derive their power from the Constitution and all members of the federal government are sworn to uphold the Constitution.