132
Opinion of the Court
need not resolve this question, however, because assuming, arguendo, that a product can be "intended to affect the structure or any function of the body" absent claims of therapeutic or medical benefit, the FDA's claim to jurisdiction contravenes the clear intent of Congress.
A threshold issue is the appropriate framework for analyzing the FDA's assertion of authority to regulate tobacco products. Because this case involves an administrative agency's construction of a statute that it administers, our analysis is governed by Chevron U. S. A. Inc. v. Natural Resources Defense Council, Inc., 467 U. S. 837 (1984). Under Chevron, a reviewing court must first ask "whether Congress has directly spoken to the precise question at issue." Id., at 842. If Congress has done so, the inquiry is at an end; the court "must give effect to the unambiguously expressed intent of Congress." Id., at 843; see also United States v. Haggar Apparel Co., 526 U. S. 380, 392 (1999); Holly Farms Corp. v. NLRB, 517 U. S. 392, 398 (1996). But if Congress has not specifically addressed the question, a reviewing court must respect the agency's construction of the statute so long as it is permissible. See INS v. Aguirre-Aguirre, 526 U. S. 415, 424 (1999); Auer v. Robbins, 519 U. S. 452, 457 (1997). Such deference is justified because "[t]he responsibilities for assessing the wisdom of such policy choices and resolving the struggle between competing views of the public interest are not judicial ones," Chevron, supra, at 866, and because of the agency's greater familiarity with the ever-changing facts and circumstances surrounding the subjects regulated, see Rust v. Sullivan, 500 U. S. 173, 187 (1991).
In determining whether Congress has specifically addressed the question at issue, a reviewing court should not confine itself to examining a particular statutory provision in isolation. The meaning—or ambiguity—of certain words or phrases may only become evident when placed in context. See Brown v. Gardner, 513 U. S. 115, 118 (1994) ("Ambiguity is a creature not of definitional possibilities but of statutory
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