Paracetamol or Acetaminophen? A History of Names Across the World

Paracetamol and acetaminophen are not two different drugs but two later names for an old compound whose history runs in three stages: it was first made in the laboratory, then eclipsed by acetanilide and phenacetin, and only later rediscovered as the key metabolite linking those earlier drugs to modern practice.

Although perhaps we should all be calling it N-acetyl-p-aminophenol, for the purposes of this article, we will refer to this medication as paracetamol.

The history of paracetamol begins not in the clinic, but in the chemistry laboratory.

In 1878, the American chemist Harmon Northrop Morse (1848-1920), working at Johns Hopkins University, published Ueber eine neue Darstellungsmethode der Acetylamidophenole (“On a New Method for the Preparation of Acetylamidophenols”). During these experiments, he observed that instead of the expected acetate salt of amidophenol, an acetylamidophenol had formed.

Morse then described the para compound as crystallising in large white prisms, readily soluble in hot water and alcohol, with a melting point of about 179°C. In modern terms, this was paracetamol, chemically characterised long before it was clinically understood. Morse did not investigate any physiological or medicinal effects. At this stage, paracetamol was simply a laboratory compound, made and described, then left without a clinical future for several decades.

Paracetamol did not move directly from Morse’s bench to the bedside. Instead, the story took a detour through the first generation of synthetic antipyretics.

In 1886, Arnold Cahn and Paul Hepp, working in the Strasbourg clinic of Adolph Kussmaul , published Das Antifebrin, ein neues Fiebermittel (“Antifebrin, a new fever remedy”). Their paper did not describe paracetamol, but acetanilide which they renamed Antifebrin . They reported antipyretic effects in animal experiments and in 24 febrile patients, presenting the drug as a powerful and inexpensive rival to Antipyrin (phenazone), the leading synthetic antipyretic of the day

The famous origin story needs caution. Cahn and Hepp state that “ a fortunate accident ” placed the preparation in their hands, but the paper itself does not tell the later elaborated tale of intestinal worms, naphthalene, and a pharmacy mix-up. The Strasbourg episode is therefore best understood as an accidental discovery whose details were expanded in later retellings. In retrospect, part of acetanilide’s clinical activity reflected metabolism to paracetamol, but Cahn and Hepp neither discovered paracetamol as a drug nor understood that metabolic relationship.

Acetanilide’s success also exposed the limits of this first generation of synthetic antipyretics. Cahn and Hepp noted cyanosis in some patients, a finding that “ at first made them somewhat uneasy ,” though they ultimately dismissed it. Later generations would recognise this as an early warning of the haematological toxicity that helped drive the search for supposedly safer related compounds.

That search led to phenacetin (acetophenetidin), developed by Otto Hinsberg with the pharmacologist Alfred Kast as a successor that appeared to preserve the antipyretic and analgesic benefits of acetanilide with fewer obvious side effects. It also fitted the industrial chemistry of the moment. Phenacetin could be made cheaply from para-nitrophenol, a by-product of the German dye industry. Later accounts disagreed on whether its discovery was chiefly a matter of luck or systematic design, but either way phenacetin became a major commercial success. Only in hindsight was it understood that much of its clinical effect led back to paracetamol.

In historical terms, this was the crucial detour. Paracetamol did not move directly from Morse’s bench to the bedside, but re-entered medicine only through the rise and problems of acetanilide and its successors.

Paracetamol did not first enter therapeutics in the late 1940s. It had already been clinically examined in the 1890s by Joseph von Mering, who was exploring relationships between chemical structure and antipyretic action among aromatic compounds. In his 1893 review, von Mering traced some of acetanilide’s action to excretion as (acetyl-)paramidophenol, then tested p-amidophenol, acetylamidophenol, and related derivatives directly. He found that acetylamidophenol had prompt antipyretic and antineuralgic effects, but appeared to retain the troublesome adverse effects of the parent amidophenol, including cyanosis and methaemoglobin-related toxicity. By contrast, he described phenacetin as an excellent antipyretic and analgesic with fewer side effects. In practical terms, paracetamol was not embraced as a new wonder drug in 1893 but was examined and passed over.

That early judgement helps explain why phenacetin, rather than paracetamol, dominated the first half of the twentieth century. Later writers have sometimes tried to read more into the 1890s than the evidence allows. Hinsberg’s 1913 retrospective is valuable because it shows that he and later Gustav Treupel were systematically exploring p-aminophenol derivatives, but it does not show that they had already established, in modern metabolic terms, that phenacetin’s effects were mediated mainly through paracetamol. At most, the work of the 1890s suggested a family resemblance among these compounds, not the decisive metabolic explanation that came later.

The real turning point came in the late 1940s. David Lester and Leon A. Greenberg showed that paracetamol occupied a key place in the metabolism of acetanilide and did not reproduce the marked methaemoglobinaemia associated with the older aniline analgesics. Soon afterwards, Brodie and Axelrod demonstrated that a major fraction of phenacetin administered to humans was converted to paracetamol. Paracetamol therefore re-emerged not as a brand-new molecule, but as the previously overlooked compound at the centre of the older drugs’ pharmacology.

The two names did not arise because chemists discovered two different drugs, nor because naming authorities deliberately set out to create a transatlantic divide. They arose because the same molecule could be shortened in two different ways from its chemical name, N-acetyl-p-aminophenol and para-acetyl-amino-phenol at a time when nonproprietary drug nomenclature was yet to be formally standardised. The split was therefore commercial first, regulatory second.

1951 – McNeil Laboratories in the United States, began developing the drug with Robert McNeil coining the name acetaminophen from N-acetyl-p-aminophenol . The brand name Tylenol was derived from the same chemical name, and the product was launched in 1955 as Children’s Tylenol Elixir.

1956 – In the United Kingdom, the drug entered clinical use in 1956 through Frederick Stearns & Co. , which used the alternative shortening paracetamol from para-acetyl-amino-phenol and marketed it as Panadol .

By the time formal naming systems matured, both names were already established in different markets. Paracetamol became the International Nonproprietary Name (INN) of the World Health Organisation, while acetaminophen remained the United States Adopted Name (USAN). What looks like scientific inconsistency is really the fossil of an earlier era of pharmaceutical marketing and nomenclature.