Why do so many prescription drug brand names carry an X or a Z, how much more often do those rare letters turn up in the names of new drugs than in everyday English, and why are the generic names behind those brands told not to start with them? A count of the brand names on the US Food and Drug Administration’s (FDA) lists of novel drug approvals gives the answer in numbers.
X and Z in the newest drug names
Of the 45 new brand name drugs the FDA approved in 2025, 19 had an X or a Z in the name, about 1 in 2.4 (our arithmetic). Z alone appeared in 13 of the 45 names, about 1 in 3.5 (our arithmetic).
“Novel drugs” here means the list kept by the FDA’s drug center of new molecular entities and new therapeutic biologics, not every approval. One 2025 drug, penpulimab-kcqx, was listed only by its generic name, so the count covers 45 brand names out of 46 approvals.
Try the old naming game
Think of a few drug brand names. Zoloft, Xanax, Zyrtec, Valtrex and Celexa are all brands still marketed in the US, according to the FDA’s Drugs@FDA database, and the newest names lean the same way: the FDA’s 2025 novel drugs include Nuzolvence, Komzifti, Romvimza, Journavx and Voyxact.
Why rare letters stand out
In ordinary English text, X and Z are near the bottom of the letter counts, which is why they catch the eye on a pill bottle.
How much more often X and Z appear
Book of Odds counted every letter in the brand names on the FDA’s novel drug lists. In the 45 brand names of 2025, Z made up 14 of 357 letters, or 3.92%, about 44 times its 0.09% share of English text in Peter Norvig’s count of Google Books (our arithmetic). X made up 6 of 357 letters, or 1.68%, about 7 times its 0.23% share of English text (our arithmetic).
The 2026 list so far, to October 20, 2026, leans the same way, a little less strongly: 13 of 47 names have an X or a Z, Z is 2.57% of the letters and X is 1.43%. Five 2026 names start with X or Z (Zanvastro, Zenbexus, Xocova, Zaynich and Zycubo), against one in all of 2025, Zegfrovy.
| Letter or count | FDA novel drug names, 2025 | 2026 to October 20 | English text |
|---|---|---|---|
| Z, share of letters | 3.92% (14 of 357) | 2.57% (9 of 350) | 0.09% |
| X, share of letters | 1.68% (6 of 357) | 1.43% (5 of 350) | 0.23% |
| Names with an X or a Z | 19 of 45 | 13 of 47 | n/a |
Counts use the FDA’s Drug Name column and the letters a to z only; two word names such as Keytruda Qlex count both words, and the one name listed without a brand and the words Co-Pack are left out.
Generic names get the opposite advice
A brand name drug has three names: a chemical name, a generic name and the brand name. Sildenafil citrate, for example, is the generic name of the drug sold as Viagra.
Generic names in the US are chosen by the United States Adopted Names (USAN) Council. The council’s naming guidelines, updated May 8, 2026, include this rule: “Avoid “z” or “x” as a beginning letter”.
Brand names go through the FDA instead. The FDA reviews a proposed brand name.
Why the FDA cares how a name sounds
The FDA’s guidance for companies on choosing brand names says sound matters: “Research has shown that linguistic characteristics can influence the way people perceive a product, particularly when it is new and unfamiliar as is the case with newly proposed proprietary names.” The same guidance suggests that a company’s own name review look for names that overstate a drug’s efficacy or safety.
The short version
Of the 45 new brand name drugs the FDA approved in 2025, 19 had an X or a Z, about 1 in 2.4, and Z showed up about 44 times as often as in English text (our arithmetic). The generic names get the opposite instruction: avoid starting with Z or X.
Last checked: 2026-10-22, against the FDA’s Novel Drug Approvals pages for 2025 and 2026 (to October 20, 2026), the AMA’s USAN naming guidelines, the FDA’s proprietary name guidance and Drugs@FDA. This article was machine-checked against the sources above and has not been expert-reviewed. Spotted an error or a newer public figure? Please tell us on our contact page with the article link and we will review it.
Sources
- S1 US Food and Drug Administration. Novel Drug Approvals for 2025. Content current as of July 8, 2026. https://www.fda.gov/drugs/novel-drug-approvals-fda/novel-drug-approvals-2025 Accessed Oct 22, 2026.
- S2 US Food and Drug Administration. Novel Drug Approvals for 2026. Content current as of October 20, 2026. https://www.fda.gov/drugs/novel-drug-approvals-fda/novel-drug-approvals-2026 Accessed Oct 22, 2026.
- S3 Peter Norvig. English Letter Frequency Counts: Mayzner Revisited. https://www.norvig.com/mayzner.html Accessed Oct 22, 2026.
- S4 American Medical Association. United States Adopted Names naming guidelines. Updated May 8, 2026. https://www.ama-assn.org/about/united-states-adopted-names-usan/united-states-adopted-names-naming-guidelines Accessed Oct 22, 2026.
- S5 US Food and Drug Administration (CDER and CBER). Best Practices in Developing Proprietary Names for Human Prescription Drug Products: Guidance for Industry. December 2020. https://www.fda.gov/media/88496/download Accessed Oct 22, 2026.
- S6 US Food and Drug Administration. Drugs@FDA: FDA-Approved Drugs, application overviews for NDA 020895 (Viagra), 019839 (Zoloft), 018276 (Xanax), 019835 (Zyrtec), 020487 (Valtrex) and 020822 (Celexa). https://www.accessdata.fda.gov/scripts/cder/daf/ Accessed Oct 22, 2026.
- S7 US Food and Drug Administration (CDER). Novel Drug Approvals at FDA. https://www.fda.gov/drugs/development-approval-process-drugs/novel-drug-approvals-fda Accessed Oct 22, 2026.