The Zook Principle Reaffirmed: Parallel State Laws Incorporating Federal Standards Are Not Preempted

1. Introduction

In Zyla Life Sciences, L.L.C. v. Wells Pharma of Houston, L.L.C. (No. 23-20533, 5th Cir. 2025), the Fifth Circuit addressed whether state unfair-competition statutes that mirror the Federal Food, Drug, and Cosmetic Act (FDCA) by incorporating its new-drug approval requirements trigger implied “obstacles-and-purposes” preemption under the Supremacy Clause.

Plaintiff-Appellant Zyla Life Sciences, LLC markets FDA-approved indomethacin suppositories nationwide. Defendant-Appellee Wells Pharma compounds indomethacin suppositories without FDA approval but registers under the FDCA’s § 353b compounding exemption. Zyla sued in six States (California, Colorado, Connecticut, Florida, South Carolina and Tennessee) under those states’ mirror-FDCA unfair-competition laws, seeking injunctions against Wells Pharma’s unapproved compounded products. The district court granted Wells Pharma’s Rule 12(b)(6) motion, holding that incorporation of the FDCA into state law creates an implied obstacle to congressional purposes. Zyla appealed.

2. Summary of the Judgment

The Fifth Circuit reversed. Relying principally on California v. Zook (336 U.S. 725 (1949)), the court held that when a state statute “makes federal law its own,” there is “no conflict in terms, and no possibility of such conflict,” so there can be no implied conflict preemption. The panel declined to extend Houston v. Moore’s outdated conflict theory, rejected Wells Pharma’s argument that federal enforcement discretion displaces parallel state statutes, and distinguished Buckman Co. v. Plaintiffs’ Legal Committee (531 U.S. 341 (2001)). The district court’s dismissal was reversed, its denial of Zyla’s leave to amend vacated, and the case remanded for further proceedings. Wells Pharma’s cross-appeal on fees was dismissed as moot.

3. Analysis

3.1. Precedents Cited

California v. Zook (336 U.S. 725, 1949): Established that a state law which “makes federal law its own” cannot conflict with federal law and therefore escapes preemption. The Fifth Circuit treated Zook as the controlling precedent for parallel incorporation statutes.
Houston v. Moore (18 U.S. 1, 1820): Early dicta holding that two “distinct wills” cannot operate concurrently. The panel explained Houston’s rationale was never broadly adopted, was repudiated by Congress in 1825, and lacks precedential force.
Wyeth v. Levine (555 U.S. 555, 2009): Confirmed that state tort suits supplementing FDCA requirements are not preempted absent express congressional command. It undercuts any broad notion that federal enforcement discretion bars parallel state action.
Buckman Co. v. Plaintiffs’ Legal Committee (531 U.S. 341, 2001): Carved out a narrow preemption rule for state-law claims that would police fraud against federal agencies, but did not disturb the principle that parallel regulation of private conduct (here, drug marketing) is permitted.
Medtronic, Inc. v. Lohr (518 U.S. 470, 1996) and Philadelphia v. New Jersey (430 U.S. 141, 1977): Reaffirmed the Supremacy Clause taxonomy—express preemption, field preemption, and conflict preemption (impossibility and purposes/obstacles).

3.2. Legal Reasoning

The court’s core reasoning proceeded in three steps:

Supremacy Clause Framework: All preemption derives from the Constitution’s Supremacy Clause. Conflict preemption exists only if state law “stands as an obstacle to the accomplishment and execution of the full purposes and objectives of Congress” (Hines v. Davidowitz, 312 U.S. 52 (1941)). “Evidence of pre-emptive purpose” is sought in the text and structure of the federal statute (CSX Transp., Inc. v. Easterwood, 507 U.S. 658 (1993)).
Zook Controls: When a state statute literally incorporates federal requirements—in this case, the FDCA’s § 505 new-drug approval bar for “new drugs”—it “makes federal law its own.” No federal purpose or objective is frustrated, because there is no divergence: identical requirements apply under state and federal law. Zook thus forecloses obstacle-and-purposes preemption.
Rejection of Alternate Theories:
- Houston’s early dicta are non-binding and rest on “arid logic” that two sovereigns cannot regulate the same subject matter at once. Modern federalism treats parallel regulation as unproblematic if no conflict emerges.
- The notion that federal enforcement discretion is upset by state action has been repeatedly rejected (García, 589 U.S. 191 (2020); Silkwood v. Kerr-McGee Corp., 464 U.S. 238 (1984)).
- Buckman is distinguishable: it bars state claims that regulate the federal–agency relationship (fraud on the FDA), not private competition regulated in parallel by state unfair-competition law.

3.3. Impact

This ruling reaffirms the viability of “mirror” statutes that incorporate federal standards, preserving state policing of unfair competition, torts and consumer protection wherever federal law also operates. It:

Confirms that state statutes which echo federal regulatory requirements will not be invalidated for obstacle preemption.
Prevents an overbroad reading of Buckman that would displace traditional state enforcement, criminal and tort rules whenever federal agencies also regulate a subject.
Strengthens state sovereignty and federalism by safeguarding concurrent state authority over health, safety, and commercial regulation, even in heavily federally regulated domains.
Guides commercial litigants and in-house counsel on the limits of implied preemption challenges to state statutes that parallel federal laws.

4. Complex Concepts Simplified

Supremacy Clause Preemption: Federal law overrides conflicting state law. But no override occurs if the state law and federal law say the same thing.
Express vs. Implied Preemption: Express preemption exists when Congress explicitly says “state law is preempted.” Implied preemption arises only if state law either (a) occupies an entire regulatory “field” or (b) conflicts—with “impossibility” (you can’t obey both) or “obstacles” (state law blocks Congress’s goals).
Zook Principle: If a state law simply adopts a federal standard wholesale, it cannot conflict with that standard and is therefore not preempted.
Enforcement Discretion: Federal agencies may choose how to enforce their statutes. Allowing states to enforce identical rules generally does not “step on” federal discretion.

5. Conclusion

The Fifth Circuit’s decision in Zyla Life Sciences v. Wells Pharma reaffirmed a century‐old principle: states may “make federal law their own” by incorporating federal requirements into state law without triggering implied obstacle‐and‐purposes preemption. By emphasizing California v. Zook and distinguishing outdated dicta and narrow precedents, the court protected the concurrent authority of states to enforce health, safety, and commercial regulations in tandem with federal statutes. The ruling fortifies the federalist balance, clarifies the scope of implied preemption under the FDCA, and preserves state unfair-competition, tort, and consumer-protection claims in federally regulated arenas.