Whereas certain drugs are sometimes known under various brand names, usually all drugs have just one generic name. One notable exception to this is the active ingredient in Tylenol, which is sometimes called acetaminophen, sometimes paracetamol. Both are abbreviations of a chemical name for the ingredient, para-acetylaminophenol. What you’ll hear depends on where you live. In the United States and Canada, acetaminophen is used. It’s also used in a few other countries, including Japan, South Korea, Hong Kong, and Iran. In the nations of the world besides those, such as in Britain, Australia, and New Zealand, paracetamol is used. Here is the Google Ngram for the two terms, from 1957-2008:
If we assume no publication biases (e.g., if Google tends to digitize more American books or not), then the above tells us that acetaminophen is the more popular of the two terms. On the other hand, the WHO uses paracetamol and officially recommends it in their list of non-proprietary drug names (yet they do list acetaminophen as a different term used in some nations; see p. 8 of this PDF).
How did this difference in nomenclature arise? As can be seen from the graph above, the terms both began to build prominence around the 1960s. The drug was first synthesized by Germans in the late 1800s, but was ignored for the next half-century at least in part because the famous pharmacologist J. von Mering mistakenly informed Bayer that it had certain negative side effects in common with earlier drugs (1, p956). But in the late 1940s and 1950s, certain American scientists studied it more closely, which led to its release to market, in a preparation with aspirin and caffeine, under the brand name Triagesic. It is noteworthy that, in their papers, these scientists simply referred to it as “N-acetyl-p-aminophenol” because a generic name had not yet been made (2, p4). It appears that, throughout the fifties, it was standard practice to refer to the drug by a chemical name. The earliest instances of ‘acetaminophen’ and ‘paracetamol’ I found were both from 1963 (3-4). In the 1960s, acetaminophen was the term used in American journals, and paracetamol the term in British journals. What likely happened was that scientists in America derived a generic name from the chemical name one way, and scientists in Britain another way, and both names stuck in their locations of origin.
(1) Brune K, Renner B, Tiegs G. Acetaminophen/paracetamol: A history of errors, failures and false decisions. Eur J Pain. 2015;19(7):953-965.
(2) Axelrod J. An unexpected life in research. Annu Rev Pharmacol Toxicol. 1988;28:1-23.
(3) Nelson E, Morioka T. Kinetics of the metabolism of acetaminophen by humans. J Pharm Sci. 1963 Sep;52(9):864-868.
(4) Smith ME, Gough J, Galpin OP. Analgesic drugs for rheumatism. BMJ. 1963 Nov 23;2(5368):1317-20.