The question presented in this case is whether the Federal Food, Drug, and Cosmetic Act precludes terminally ill cancer patients from obtaining Laetrile, a drug not recognized as "safe and effective" within the meaning of § 201(p)(1) of the Act, 52 Stat. 1041, as amended, 21 U.S.C. § 321(p)(1).

* Section 505 of the Federal Food, Drug, and Cosmetic Act, 52 Stat. 1052, as amended, 21 U.S.C. § 355, prohibits interstate distribution of any "new drug" unless the Secretary of Health, Education, and Welfare approves an application supported by substantial evidence of the drug's safety and effectiveness.1 As defined in § 201(p)(1) of the Act, 21 U.S.C. § 321(p)(1), the term "new drug" includes

"[a]ny drug . . . not generally recognized, among experts qualified by scientific training and experience to evaluate the safety and effectiveness of drugs, as safe and effective for use under the conditions prescribed, recommended, or suggested in the labeling . . . ." Exemptions from premarketing approval procedures are available for drugs intended solely for investigative use2 and drugs qualifying under either of the Act's two grandfather provisions.3

In 1975, terminally ill cancer patients and their spouses brought this action to enjoin the Government from interfering with the interstate shipment and sale of Laetrile, a drug not approved for distribution under the Act.4 Finding that Laetrile, in proper dosages, was nontoxic and effective, the District Court ordered the Government to permit limited purchases of the drug by one of the named plaintiffs. 399 F. Supp. 1208, 1215 (WD Okl. 1975).5 On appeal by the Government, the Court of Appeals for the Tenth Circuit did not disturb the injunction. However, it instructed the District Court to remand the case to the Food and Drug Administration for determination whether Laetrile was a "new drug" under § 201(p)(1), and, if so, whether it was exempt from premarketing approval under either of the Act's grandfather clauses. 542 F.2d 1137 (1976).

After completion of administrative hearings,6 the Commissioner issued his opinion on July 29, 1977. 42 Fed.Reg. 39768 (1977). He determined first that no uniform definition of Laetrile exists; rather, the term has been used generically for chemical compounds similar to, or consisting at least in part of, amygdalin, a glucoside present in the kernels or seeds of most fruits. Id., at 39770-39772. The Commissioner further found that Laetrile in its various forms constituted a "new drug" as defined in § 201(p)(1) of the Act because it was not generally recognized among experts as safe and effective for its prescribed use. See 42 Fed.Reg. 39775-39787 (1977). In so ruling, the Commissioner applied the statutory criteria delineated in Weinberger v. Hynson, Westcott & Dunning, Inc., 412 U.S. 609, 629-630, 73 S.Ct. 2469, 2483, 37 L.Ed.2d 207 (1973), and concluded that there were no adequate well-controlled scientific studies of Laetrile's safety or effectiveness. 42 Fed.Reg. 39775-39787 (1977).7

Having determined that Laetrile was a new drug, the Commissioner proceeded to consider whether it was exempt from premarketing approval under the 1938 or 1962 grandfather provisions. On the facts presented, the Commissioner found that Laetrile qualified under neither clause. See id., at 39787-39795. First, there was no showing that the drug currently known as Laetrile was identical in composition or labeling to any drug distributed before 1938. See 21 U.S.C. § 321(p)(1); n. 3, supra. Nor could the Commissioner conclude from the evidence submitted that, as of October 9, 1962, Laetrile in its present chemical composition was commercially used or sold in the United States, was generally recognized by experts as safe, and was labeled for the same recommended uses as the currently marketed drug. See § 107(c)(4), 76 Stat. 789; n. 3, supra.

On review of the Commissioner's decision, the District Court sustained his determination that Laetrile, because not generally regarded as safe or effective, constituted a new drug under § 201(p)(1). 438 F.Supp. 1287, 1293-1294 (WD Okl. 1977). The court also approved the Commissioner's denial of an exemption under the 1938 grandfather clause. However, concluding that the record did not support the Commissioner's findings as to the 1962 grandfather provision, the District Court ruled that Laetrile was entitled to an exemption from premarketing approval requirements. Id., at 1294-1298. Alternatively, the court held that, by denying cancer patients the right to use a nontoxic substance in connection with their personal health, the Commissioner had infringed constitutionally protected privacy interests. Id., at 1298-1300.

The Court of Appeals addressed neither the statutory nor the constitutional rulings of the District Court. Rather, the Tenth Circuit held that "the 'safety' and 'effectiveness' terms used in the statute have no reasonable application to terminally ill cancer patients." 582 F.2d 1234, 1236 (1978). Since those patients, by definition, would "die of cancer regardless of what may be done," the court concluded that there were no realistic standards against which to measure the safety and effectiveness of a drug for that class of individuals. Id., at 1237. The Court of Appeals therefore approved the District Court's injunction permitting use of Laetrile by cancer patients certified as terminally ill. However, presumably because the Commissioner had found some evidence that Laetrile was toxic when orally administered, see 42 Fed.Reg. 39786-39787 (1977), the Court of Appeals limited relief to intravenous injections for patients under a doctor's supervision. 582 F.2d, at 1237. In addition, the court directed the FDA to promulgate regulations "as if" the drug had been found " 'safe' and 'effective' " for terminally ill cancer patients. Ibid.

We granted certiorari, 439 U.S. 1127, 99 S.Ct. 1042, 59 L.Ed.2d 87 (1979), and now reverse.

The Federal Food, Drug, and Cosmetic Act makes no special provision for drugs used to treat terminally ill patients. By its terms, § 505 of the Act requires premarketing approval for "any new drug" unless it is intended solely for investigative use or is exempt under one of the Act's grandfather provisions. See nn. 2, 3, supra. And § 201(p)(1) defines "new drug" to encompass "[a]ny drug . . . not generally recognized . . . as safe and effective for use under the conditions prescribed, recommended, or suggested in the labeling." See supra, at 546-547.

When construing a statute so explicit in scope, a court must act within certain well-defined constraints. If a legislative purpose is expressed in "plain and unambiguous language, . . . the . . . duty of the courts is to give it effect according to its terms." United States v. Lexington Mill & Elevator Co., 232 U.S. 399, 409, 34 S.Ct. 337, 340, 58 L.Ed. 658, 662 (1914). See Andrus v. Sierra Club, 442 U.S. 347, 99 S.Ct. 2335, 60 L.Ed.2d 943 (1979). Exceptions to clearly delineated statutes will be implied only where essential to prevent "absurd results" or consequences obviously at variance with the policy of the enactment as a whole. Helvering v. Hammel, 311 U.S. 504, 510-511, 61 S.Ct. 368, 371, 85 L.Ed. 303 (1941). See TVA v. Hill, 437 U.S. 153, 187-188, 98 S.Ct. 2279, 2298, 57 L.Ed.2d 117 (1978); United States v. Key, 397 U.S. 322, 324-325, 90 S.Ct. 1049, 1051, 25 L.Ed.2d 340 (1970); United States v. American Trucking Assns., 310 U.S. 534, 543-544, 60 S.Ct. 1059, 1063-1064, 84 L.Ed. 1345 (1940). In the instant case, we are persuaded by the legislative history and consistent administrative interpretation of the Act that no implicit exemption for drugs used by the terminally ill is necessary to attain congressional objectives or to avert an unreasonable reading of the terms "safe" and "effective" in § 201(p)(1).

Nothing in the history of the 1938 Food, Drug, and Cosmetic Act, which first established procedures for review of drug safety, or of the 1962 Amendments, which added the current safety and effectiveness standards in § 201(p)(1),8 suggests that Congress intended protection only for persons suffering from curable diseases. To the contrary, in deliberations preceding the 1938 Act, Congress expressed concern that individuals with fatal illnesses, such as cancer, should be shielded from fraudulent cures. See, e. g., 79 Cong.Rec. 5023 (1935) (remarks of Sen. Copeland, sponsor of the Act); 83 Cong.Rec. 7786-7787, 7789 (1938) (remarks of Reps. Phillips and Lea). Similarly, proponents of the 1962 Amendments to the Act, including Senator Kefauver, one of the bill's sponsors, indicated an understanding that experimental drugs used to treat cancer "in its last stages" were within the ambit of the statute. See, e. g., 108 Cong.Rec. 17399 (1962) (remarks of Sen. Kefauver); id., at 17401 (comments of Sen. Eastland). That same understanding is reflected in the Committee Reports on the 1962 Amendments. Both Reports note with approval the FDA's policy of considering effectiveness when passing on the safety of drugs prescribed for "life-threatening disease."9

In implementing the statutory scheme, the FDA has never made exception for drugs used by the terminally ill. As this Court has often recognized, the construction of a statute by those charged with its administration is entitled to substantial deference. Board of Governors of FRS v. First Lincoln wood Corp., 439 U.S. 234, 248, 99 S.Ct. 505, 513, 58 L.Ed.2d 484 (1978); Bayside Enterprises, Inc. v. NLRB, 429 U.S. 298, 304, 97 S.Ct. 576, 581, 50 L.Ed.2d 494 (1977); Udall v. Tallman, 380 U.S. 1, 16, 85 S.Ct. 792, 801, 13 L.Ed.2d 616 (1965). Such deference is particularly appropriate where, as here, an agency's interpretation involves issues of considerable public controversy, and Congress has not acted to correct any misperception of its statutory objectives. See Red Lion Broadcasting Co. v. FCC, 395 U.S. 367, 381, 89 S.Ct. 1794, 1801, 23 L.Ed.2d 371 (1969); Zemel v. Rusk, 381 U.S. 1, 11-12, 85 S.Ct. 1271, 1278, 14 L.Ed.2d 179 (1965).10 Unless and until Congress does so, we are reluctant to disturb a longstanding administrative policy that comports with the plain language, history, and prophylactic purpose of the Act.

In the Court of Appeals' view, an implied exemption from the Act was justified because the safety and effectiveness standards set forth in § 201(p)(1) could have "no reasonable application" to terminally ill patients. 582 F.2d, at 1236. We disagree. Under our constitutional framework, federal courts do not sit as councils of revision, empowered to rewrite legislation in accord with their own conceptions of prudent public policy. See Anderson v. Wilson, 289 U.S. 20, 27, 53 S.Ct. 417, 420, 77 L.Ed. 1004 (1933). Only when a literal construction of a statute yields results so manifestly unreasonable that they could not fairly be attributed to congressional design will an exception to statutory language be judicially implied. See TVA v. Hill, 437 U.S., at 187-188, 98 S.Ct., at 2298. Here, however, we have no license to depart from the plain language of the Act, for Congress could reasonably have intended to shield terminal patients from ineffectual or unsafe drugs.

A drug is effective within the meaning of § 201(p)(1) if there is general recognition among experts, founded on substantial evidence, that the drug in fact produces the results claimed for it under prescribed conditions. See Weinberger v. Hynson, Westcott & Dunning, Inc., 412 U.S., at 629-634, 93 S.Ct., at 2483-2485; n. 7, supra. Contrary to the Court of Appeals' apparent assumption, see 582 F.2d, at 1236, effectiveness does not necessarily denote capacity to cure. In the treatment of any illness, terminal or otherwise, a drug is effective if it fulfills, by objective indices, its sponsor's claims of prolonged life, improved physical condition, or reduced pain. See 42 Fed.Reg. 39776-39786 (1977).

So too, the concept of safety under § 201(p)(1) is not without meaning for terminal patients. Few if any drugs are completely safe in the sense that they may be taken by all persons in all circumstances without risk.11 Thus, the Commissioner generally considers a drug safe when the expected therapeutic gain justifies the risk entailed by its use.12 For the terminally ill, as for anyone else, a drug is unsafe if its potential for inflicting death or physical injury is not offset by the possibility of therapeutic benefit. Indeed, the Court of Appeals implicitly acknowledged that safety considerations have relevance for terminal cancer patients by restricting authorized use of Laetrile to intravenous injections for persons under a doctor's supervision. See 582 F.2d, at 1237; supra, at 551.

Moreover, there is a special sense in which the relationship between drug effectiveness and safety has meaning in the context of incurable illnesses. An otherwise harmless drug can be dangerous to any patient if it does not produce its purported therapeutic effect. See 107 Cong.Rec. 5640 (1961) (comments of Sen. Kefauver). But if an individual suffering from a potentially fatal disease rejects conventional therapy in favor of a drug with no demonstrable curative properties, the consequences can be irreversible.13 For this reason, even before the 1962 Amendments incorporated an efficacy standard into new drug application procedures, the FDA considered effectiveness when reviewing the safety of drugs used to treat terminal illness. See nn. 8, 9, supra. The FDA's practice also reflects the recognition, amply supported by expert medical testimony in this case, that with diseases such as cancer it is often impossible to identify a patient as terminally ill except in retrospect.14 Cancers vary considerably in behavior and in responsiveness to different forms of therapy. See 42 Fed.Reg. 39777 (1977).15 Even critically ill individuals may have unexpected remissions and may respond to conventional treatment. Id., at 39777, 39805. Thus, as the Commissioner concluded, to exempt from the Act drugs with no proved effectiveness in the treatment of cancer "would lead to needless deaths and suffering among . . . patients characterized as 'terminal' who could actually be helped by legitimate therapy." Id., at 39805.

It bears emphasis that although the Court of Appeals' ruling was limited to Laetrile, its reasoning cannot be so readily confined. To accept the proposition that the safety and efficacy standards of the Act have no relevance for terminal patients is to deny the Commissioner's authority over all drugs, however toxic or ineffectual, for such individuals. If history is any guide, this new market would not be long overlooked. Since the turn of the century, resourceful entrepreneurs have advertised a wide variety of purportedly simple and painless cures for cancer, including liniments of turpentine, mustard, oil, eggs, and ammonia; peat moss; arrangements of colored floodlamps; pastes made from glycerin and limburger cheese; mineral tablets; and "Fountain of Youth" mixtures of spices, oil, and suet.16 In citing these examples, we do not, of course, intend to deprecate the sincerity of Laetrile's current proponents, or to imply any opinion on whether that drug may ultimately prove safe and effective for cancer treatment. But this historical experience does suggest why Congress could reasonably have determined to protect the terminally ill, no less than other patients, from the vast range of self-styled panaceas that inventive minds can devise.

We note finally that construing § 201(p)(1) to encompass treatments for terminal diseases does not foreclose all resort to experimental cancer drugs by patients for whom conventional therapy is unavailing. Section 505(i) of the Act, 21 U.S.C. § 355(i), exempts from premarketing approval drugs intended solely for investigative use if they satisfy certain preclinical testing and other criteria.17 An application for clinical testing of Laetrile by the National Cancer Institute is now pending before the Commissioner. Brief for United States 35 n. 23. That the Act makes explicit provision for carefully regulated use of certain drugs not yet demonstrated safe and effective reinforces our conclusion that no exception for terminal patients may be judicially implied. Whether, as a policy matter, an exemption should be created is a question for legislative judgment, not judicial inference.

The judgment of the Court of Appeals is reversed, and the case is remanded for further proceedings consistent with this opinion.18

So ordered.