In the case of Food and Drug Administration v. Brown and Williamson Tobacco Corp. (529 U.S. 120 (2000)), the FDA decided that they had the authority to regulate tobacco. The US Supreme Court disagreed. The US Supreme Court found that if you look at the facts, there is no way that tobacco could possibly be considered safe under FDA's guidelines. Therefore if FDA were to regulate tobacco, they would have no choice but to ban it. The Court looked to the entirety of Congress' actions and statements about tobacco, and found that there was no Congressional intent to ban tobacco. Therefore, under the Chevron Doctrine (See Chevron U.S.A. Inc. v. Natural Resources Defense Council (467 U.S. 837 (1984))), since Congressional intent is clear on the issue, there is no room for Agency interpretation. Btw, if Congressional intent is ambiguous, then the courts will defer to Agency interpretation, as long as that interpretation is 'reasonable'.