Three doors, three records. Each with its date.
Which route does the record show?
Section 503A names three routes for a bulk substance (21 U.S.C. 353a(b)(1)(A)(i)). One is enough; each line says what its record shows.
| Door | What the record shows | Source, read |
|---|---|---|
| A USP or NF monographNot recorded | Not established: the registry holds no USP monograph title for this record. Silence is not proof that none exists. | FDA's substance registry (GSRS) Read October 19, 2026 |
| A component of a drug marketed under an FDA applicationNot recorded | Drugs@FDA lists no application containing it by name or UNII. | openFDA Drugs@FDA Read October 19, 2026 |
| The 503A bulks list, 21 CFR 216.23(a)Route recorded | Named at 21 CFR 216.23(a): "Squaric acid dibutyl ester (for topical use only)". | 21 CFR 216.23 Read October 21, 2026 |
| Withdrawn drug products21 CFR 216.24 | Not named in 21 CFR 216.24. | 21 CFR 216.24 Read October 19, 2026 |
A recorded route is one condition of section 503A, not all of them: the maker must be registered, each bulk substance must come with a valid certificate of analysis, and the preparation must meet the rest of the section. Whether a substance may be compounded is the pharmacist's call, on the facts of the preparation. This page reads FDA's documents; it decides nothing.
On FDA's lists. As printed by FDA on May 14, 2026, not a ruling.
Is it on the 503A bulks list or in a category?
On the 503A bulks list: "Squaric acid dibutyl ester (for topical use only)". Named in the regulation as a bulk drug substance that can be used in compounding under section 503A(b)(1)(A)(i)(III).
A vote of FDA's Pharmacy Compounding Advisory Committee (PCAC) is advice to FDA. The 503A bulks list is developed through regulations (21 U.S.C. 353a(b)(1)(A)(i)(III) and (c)) and printed at 21 CFR 216.23, so it changes only by rulemaking.
Watch Squaric acid dibutyl ester. One email when it changes.
How do I hear about a change?
An email when FDA's documents change for this substance. You confirm once.
Make a dated receipt for one lot.
Squaric acid dibutyl ester, the maker its certificate names and the lot, with every source dated.