"Acyclovir Impurity B" is the name for guanine used by a handful of relatively low-traffic websites. Googling the term shows this. It is nearly always with a capital letter at the beginning of each word, which is another slightly strange thing, since it is a generic name, and therefore should be lower case except for the "B".

What seems especially remarkable is that This page was the relevant hit that was highest up on Page One of Google when I searched for "2-amino-1,9-dihydro-6H-purin-6-one" without quotes. The page linked to states the name as "Acyclovir Impurity B (sic)" and only in the fine print below does it mention any other names, where it says, "Synonyms-



This is another website that confirms that "Acyclovir Impurity B" is a name for guanine.

There's nothing to indicate that guanine is ever referred by this name in Wikipedia. The closest I could find in Wikipedia by Googling "acyclovir impurity B" was in the entry for acyclovir where it is written

The aciclovir metabolite 9-Carboxymethoxymethylguanine (9-CMMG) has been shown to play a role in neurological adverse events, particularly in older people and those with reduced renal function.[34][35][36]

which contains the string "guanine".

It looks like many to all sites using the term "Acyclovir Impurity B" are selling it.

This paper says that guanine is the major impurity of acyclovir, but doesn't explain why some others selling it avoid using the term "guanine". It also doesn't explain what the "B" means (doesn't that suggest that it is the second most major impurity?).

PubChem has this page that lists 122 "depositor-supplied synonyms" for "guanine". Included in the list are strange names like "mearlmaid" and "pearl essence" and various variations on "Acyclovir Impurity B" such as "Aciclovir EP Impurity B" though not the exact same name, as far as I could tell. I mean, to me, adding "EP" makes it a different name, though I could be wrong because I don't know what "EP" means.

Googling "mearlmaid" brought up the Wikipedia article on guanine, but searching the article for the word "mearlmaid" didn't detect it, so I guess it's in the meta text or whatever it's called.

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    $\begingroup$ I assume it may be just operational sequentional naming of unknown impurities on HPLC chromatograms, until the identity was determined later. Then others may just refer to an original paper $\endgroup$
    – Poutnik
    Dec 21, 2022 at 7:59
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    $\begingroup$ If you want to see all the synonyms used for a compound, check PubChem. You'll note there are many, many terms. I suspect some of the low-traffic websites take every synonym in Wikipedia and list them in case someone is searching for "Mearlmaid" in which case I see a sales link in the top list. $\endgroup$ Dec 21, 2022 at 13:37
  • $\begingroup$ @GeoffHutchison Awesome link, have added it to the question. I also added some text clarifying that I searched for "2-amino-1,9-dihydro-6H-purin-6-one" without quotes using Google and the top relevant hit was "Acyclovir Impurity B". $\endgroup$ Dec 21, 2022 at 16:01

1 Answer 1


Guanine is guanine. It was discovered centuries ago and named as such. No decent scientific paper will accept Acyclovir Impurity B as its synonym (without its context). In the pharmaceutical industry, however, this label is acceptable. EP stands for European Pharmacopia (set of rules for the pharma industry). The nomenclature A, B, C, D, there makes sense. Any medicine/drug that gets commercially prepared undergoes a very stringent chemical analysis for quality control.

In fact, an analytical expert who is able to analyze known and unknown peaks in a pharmaceutical product is highly valued (very high salary). In this case, acyclovir is the active pharmaceutical (AP) and then if someone analyzes it, e.g., on a liquid chromatography-mass spectrometry system, he/she might see signals (peaks) from other impurities. In order to assign those impurities in the prepared product analysis result, one can run an impurity standard of that particular AP. Instead of having a mouthful of impurity names in the standard, it is easier to designate those peaks by English letters "AP Impurity A", "AP Impurity B" etc. This is just pharma jargon, so there is nothing unusual about it. Guanine will remain guanine.

  • $\begingroup$ That is plausible. It makes perfect sense. On the other hand, I think the answer would be better still if it had a citation. $\endgroup$ Dec 23, 2022 at 15:02
  • $\begingroup$ @MatthewChristopherBartsh, The hint is right there! EP = "The European Pharmacopoeia (Ph. Eur.) is a single reference work for the quality control of medicines." Chapter 5.10. CONTROL OF IMPURITIES IN SUBSTANCES FOR PHARMACEUTICAL USE. $\endgroup$
    – AChem
    Dec 23, 2022 at 16:12

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