Why is guaifenesin so difficult to spell?

Even among drugs names that are often difficult to pronounce or spell, guaifenesin stands out for tripping up more students on spelling in exams than other drug names. Why is “guaifenesin” spelt this way? 

Breaking “guaifenesin” up into “guai” and “fenesin” may help us to remember how to spell the word. It is the “guai” that in particular seems unnatural in English and is difficult to spell. Perhaps understanding the origins of the “guai” in “guaifenesin” can help us to remember how to spell the word.

The “guai” in “guaifenesin” comes from the word “guaiac”. Guaiac has been an English word since at least 1558, some say 1533. It is the common name for trees of the genus Guaiacum.  The word originates from the Maipurean language spoken by the native Taínos people of the Bahamas. “Guaiac” has the honour of being the first American language word adopted into the English language.  The guaiac is famous for being the source of the hardest wood known. The resin and bark of the guaiac were also used in traditional medicine for coughs and various other conditions.  Guaifenesin is the active compound in the treatment of coughs isolated from guaiac resin and bark.

Guaifenesin was also formerly spelt “guaiphenesin”. It is one of the few drugs for which the American contraction of “ph” to “f” is now adopted for the official international nonproprietary name of the drug. The chemical name for guaifenesin is glyceryl guaiacolate.

Interestingly, guaiac resin also made another significant contribution to medicine. A phenolic compound derived from guaiac tree resin has also been used in the faecal occult blood test (FOBT).  The presence of haeme from blood causes this compound to form a coloured product when exposed to hydrogen peroxide.