Hippietrail tagged this as an RfV over a year ago, based on the discrepancy between our definition and Wikipedia's. I did some research and found the following:
"Patent medicine is an English term which refers to the registration with the British Patent Office of a given compound as a medicine. In the United States it refers to those drugs which are sold without a prescription. Thus, in the United States, the term "patent medicine" is a misnomer since no patent is required." Robert H. Coombs, Lincoln J. Fry, Patricia G. Lewis, Socialization in Drug Abuse (1976), p. 10.
"The term "patent medicine" originated in England and referred to "patents of royal favor" that kings granted to their bootmakers, tailors, and medicine makers. By definition, true patent medicines revealed their ingredients on their labels as a condition of maintaining their patent on that formulation. The so-called "patent medicines" produced in America were actually proprietary drugs in which the unique shape and color of the bottles along with the label designs were protected by trademark. The actual ingredients within the bottles, however, were kept secret — a practice that only added to the medicine's mystique. "Patent medicine" became a misused term due to the lack of distinction between patented and unpatented medicines in ads and on store shelves." Charles R. Whitlock, Ben Chandler, Mediscams: Dangerous Medical Practices and Health Care Frauds, (2003), p. 39.
"Just a word as to the distinction made between proprietary medicines and "patent medicines." Strictly speaking, practically all nostrums on the market are proprietary medicines and but very few are true patent medicines. A patent medicine, in the legal sense of the word, is a medicine whose composition or method of making, or both, has been patented. Evidently, therefore, a patent medicine is not a secret preparation because its composition must appear in the patent specifications. Nearly every nostrum, instead of being patented, is given a fanciful name and that name is registered at Washington; the name thus becomes the property of the nostrum exploiter for all time. While the composition of the preparation, and the curative effects claimed for it, may be changed at the whim of its owner, his proprietorship in the name remains intact. As has been said, a true patent medicine is not a secret preparation; moreover, the product becomes public property at the end of seventeen years. As the term "patent medicine" has come to have a definite meaning to the public, this term is used in its colloquial sense throughout the book. That is to say, all nostrums advertised and sold direct to the public are referred to as "patent medicines"; those which are advertised directly only to physicians are spoken of as 'proprietaries.'" American Medical Association, Nostrums and Quackery (1921), p. 6.
So it seems that what we have here is a UK/U.S. usage divide, with the U.S. usage clearly divorced from ownership of an actual patent. bd2412T 22:59, 4 August 2008 (UTC)
According to the OED: "a proprietary medicine manufactured under patent and available without prescription". You're right that in the US it seems to have become a synonym for simply "non-prescription medicine". Is it ever used this way in the UK I wonder? I don't think I've ever heard it. Ƿidsiþ 09:36, 5 August 2008 (UTC)
Although the OED has only the "official" meaning, the term is sometimes used in its wider sense in the UK (certainly in Northern England), but this is probably just "patent" in its wider sense of "In extended use: to which a person has a proprietary claim. Also: special for its purpose; ingenious, well-contrived" (OED). Dbfirs 09:45, 28 November 2008 (UTC)
In the US, there is no necessary connection between a drug being patented and being a prescription drug or being sold over the counter or being a "controlled substance". In the US patent medicine is dated (19th-early 20th century, I think), but referred to non-prescription medicines. DCDuringTALK 11:53, 28 November 2008 (UTC)