What the letters after a drug name mean, from ER and XL to ODT and HCl

FDA's National Drug Code Directory listed 3,589 generic extended-release tablets and capsules for prescription use on Nov. 4, 2026. In 3,110 of those listings, 87 percent, the product name carried no release letters at all, no ER, XL or SR. Among the 497 listings filed under new drug applications in the same dosage forms, the most common tag was XR, on 124. Those counts come from GxRadar's analysis of the openFDA NDC Directory data¹, which publish what companies submit when they list a drug with FDA.

That gap says something about where the meaning of those letters lives. A short suffix such as XL or ODT is part of a product's name, chosen by the company that markets it. The fact that a tablet releases its drug slowly is recorded elsewhere, in a dosage form term FDA keeps in a controlled vocabulary.

Readers meet these letters on bottles, pharmacy stickers and insurance lists, and they sometimes change when a prescription moves from a brand to a generic. This explainer sorts out which letters FDA has defined, what the official dosage form names say, why generic names often drop the letters, and what HCl and other salt words mean. Questions about whether two versions can be used interchangeably belong with a pharmacist or prescriber.

The letters FDA has written down

FDA's 2020 guidance on proprietary names² calls these add-ons modifiers and says they are usually placed at the end of a root name, though oral contraceptives often put them in front. It is blunt about the record. "Inconsistencies in the use of modifiers and the absence of a standardized meaning for some modifiers that are meant to convey information about a product have been a source of confusion to end users," the guidance says, and that confusion "has led to medication errors, such as dispensing and administering wrong formulation, wrong dose, wrong strength, or wrong frequency of administration."

Errors also happen "within the same product line if the distinguishing modifier is omitted or disregarded when a product is prescribed or dispensed," it adds. To help, the guidance's Appendix A lists modifiers that, used with these meanings, have not been a source of confusion.

SR, XL, CR and CD are not on that list, though all four appear on products in pharmacies today. The guidance does not assign them meanings, and GxRadar does not either.

What the official dosage form says

Every listed drug carries a dosage form drawn from FDA's structured product labeling vocabulary³, terms such as TABLET, EXTENDED RELEASE; CAPSULE, DELAYED RELEASE; and TABLET, ORALLY DISINTEGRATING, each tied to a National Cancer Institute concept code. The list has no entry for XL or SR.

The generic name itself never encodes release. USAN rules bar prefixes that evoke "the name of the sponsor, dosage form, duration of action or rate of drug release," the naming guidelines¹⁰ say, citing "dura" as an example. Definitions for the dosage form terms are short. An extended-release tablet is "a solid dosage form containing a drug which allows at least a reduction in dosing frequency as compared to that drug presented in conventional dosage form," according to the NCI Thesaurus entry⁴ for the FDA term. A delayed-release tablet "releases a drug (or drugs) at a time other than promptly after administration," and "enteric-coated articles are delayed release dosage forms," per the matching entry⁵. For orally disintegrating tablets, FDA's 2008 guidance⁶ adds a number, recommending an in-vitro disintegration time of approximately 30 seconds or less.

When GxRadar matched suffix letters in prescription product names against the dosage forms those products were listed under in the NDC Directory¹, the letters lined up closely with release type, though not perfectly.

Counts are human prescription product listings as of Nov. 4, 2026, matched on whole-word letters in the brand name or brand-name suffix fields. A listing is one company's product, so a single drug can appear many times.

Why generic names often skip the letters

A generic is listed under its established name, and FDA notes in its 2022 guidance on container labels⁷ that "established names for drug products typically include the dosage form." In the NDC Directory, a generic's name field usually holds just the molecule name, while the release type sits in the separate dosage form field; on the printed label, the established name carries the dosage form in words, such as "extended-release tablets." Of the 3,589 generic extended-release listings, 216 spelled out "extended-release" or "extended release" in the name field itself.

FDA's label guidance⁷ also says a frequency statement, such as "once daily" for an extended-release oral product dosed once a day, may be considered for the main panel of a container label when there is concern that frequency errors could occur.

How FDA compares extended-release generics with their brands is covered in its piece on extended-release and immediate-release pills. Whether a particular generic is rated as substitutable for a particular brand appears in the Orange Book's therapeutic equivalence codes, and a pharmacist can say which version a prescription calls for. Drug pages such as metoprolol succinate ER, omeprazole DR and duloxetine DR show how GxRadar labels release forms.

HCl and other salt words

Not every word after a drug name is about release. Many are salts, in which the drug molecule is paired with an acid or a base, such as hydrochloride or sodium. USAN rules give salts and esters two-word names, with the active part first in most cases, as in "oxacillin sodium," according to the USAN naming guidelines¹⁰. HCl stands for hydrochloride. FDA's container label guidance says salt abbreviations should be avoided at the beginning of a name, citing "Na Salicylate" for sodium salicylate, but "the abbreviation HCl for hydrochloride at the end of a name does not raise concerns."

Newer products may drop the salt from the title entirely. Under the USP Salt Policy, effective May 1, 2013, product monograph titles use the active moiety, "newdrug tablets" instead of "newdrug hydrochloride tablets," and strength is stated in terms of the active moiety, FDA explains in its 2015 guidance on salt drug substances⁸. The salt still has to appear on the label. GxRadar's companion piece on salt forms and drug names and its explainer on what USP means go further.

Shorthand for whole drug names is a different matter. FDA's container label guidance⁷ tells sponsors to avoid error-prone abbreviations "for product names, doses, and strength designations," and the Institute for Safe Medication Practices advises against abbreviating drug names at all. Its 2024 list of error-prone abbreviations⁹ records HCTZ, meant as hydrochlorothiazide, being mistaken for hydrocortisone, and HCT, meant as hydrocortisone, mistaken for hydrochlorothiazide. GxRadar's explainer on look-alike and sound-alike drug names covers related name-confusion risks.

A suffix can also be lost in repackaging. In 2016, according to FDA's enforcement records¹¹, a repackager recalled venlafaxine extended-release tablets after labeling them as the immediate-release formulation. FDA classed it Class III.