I like the idea, with a couple comments:
(1) this will be country dependant. Not sure whether CHEBI considers this, but the same molecule will have different names, brand names etc across countries.
For example aspirin is a generic name in the USA, while it is trademarked in Canada.
https://www.rxlist.com/aspirin/generic-drug.htm shows that "Aspirin is available under the following different brand names: Zorprin, Bayer Buffered Aspirin "

(2) Maybe there is something different between "aspirin", "paracetamol" etc and the drug brand name from the manufacturer? For example Tylenol or Panadol for the latter (in addition to aspirin examples above) Based on your proposed definition of the property would it be fair to say "aspirin" and "paracetamol" are synonyms, while "Aspirin" (where trademarked), "Zorprin", "Tylenol" etc are brand names?

that's a good point. I would lean on having a country-agnostic definition for this high-level term, as at this point I don't think the information about country specificity is available in ChEBI (I think it just slices by language, and not country)

I wonder if this also means we should be grouping this under "layperson synonym" and not the top-level synonym type

Not doing country specific for the generic property sounds good to me.

I like the idea of 'layperson synonym' but the label bothers me, e.g. paracetamol/acetaminophen is really the name used by pharmacists etcs, see https://en.wikipedia.org/wiki/Paracetamol#Naming.

What about something similar to https://en.wikipedia.org/wiki/International_nonproprietary_name? I think that may be what we are looking for here.

I like the idea of 'layperson synonym' but the label bothers me, e.g. paracetamol/acetaminophen is really the name used by pharmacists etcs, see https://en.wikipedia.org/wiki/Paracetamol#Naming.

That's a really interesting link that could lead to a whole swath of other requests here. I also see your point about it not being aligned well with "layperson synonym"

INN is also used as a different ChEBI synonym, so I think that is definitely the top of the list. But I would be interested to add the Austrailian Approved Name, British Approved name, United States Adopted Name, Japanese Accepted name, etc. to give additional context when desired.

No need to assign me; maybe share the issue once in the ontology metadata channel, for greater visibility, and if you don't hear anything negative in 72 hours, just make a PR. I have no larger concerns, other than the fact that the border between "synonym" and other kinds of relations are becoming pretty fuzzy here.

I am not sure how many clinicians would consider the brand name a synonym of the generic name or even the chemical name, they are pretty different things, at least I thought so. But lets get some others to weigh in, maybe even someone from Chebi and PRO (@nataled).

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Sorry if this is too pure, but it says for example here that synonyms are supposed to be reflexive. (We can presume that gets modified to the extent a synonym is only close or related, as opposed to an exact synonym. But the concept still applies IMHO.)

I'm not seeing how Advil someSynonym ibuprofen is reflexive. One is clearly a subset of the other, and they have different characteristics and semantic applications. (All Advils are ibuprofen, but not all ibuprofens are Advils. The day that an Advil has an unfortunate manufacturing defect that ibuprofen is not suppose to have, considering them a synonym will be clearly a bad idea.

What am I missing?

Some issues:

1. Neither paracetamol nor acetaminophen is a brand name. I just want to clarify that because it seems like that issue is being confused with brand name
2. The different brand names issues is an important one. Minimally, brand will differ across countries, but they vary even within a country
3. I've seen in the past where everyone admixture of active ingredients that contains acetaminophen is conferred as a "brand name" on acetaminophen. For example 'Vicodin'. Well, 'Vicodin' is NOT NOT NOT NOT NOT NOT a brand name of acetaminophen. It is quite specifically and definitively a brand name of a combination of hydrocodone and acetaminophen. That for sure would blow apart any sense of reflexiveness. The same issue exists in spades with all the crazy cold/cough formulations that involve acetaminophen. 'Dayquil' is also NOT a brand name of 'acetaminophen'