delivered the opinion of the Court.
The policy of sentencing drug offenders based on the amount of drugs involved, straightforward enough in its simplest formulation, gives rise to complexities, requiring us again to address the methods for calculating the weight of LSD sold by a drug trafficker. We reject petitioner’s contention that the revised system for determining LSD amounts under the United States Sentencing Guidelines requires reconsideration of the method used to determine *286 statutory minimum sentences, and we adhere to our former decision on the subject.
I
LSD (lysergic acid diethylamide) is such a powerful narcotic that the average dose contains only 0.05 milligrams of the pure drug. The per-dose amount is so minute that in most instances LSD is transferred to a carrier medium and sold at retail by the dose, not by weight. In the typical case, pure LSD is dissolved in alcohol or other solvent, and the resulting solution is applied to paper or gelatin. The solvent evaporates; the LSD remains. The dealer cuts the paper or gel into single-dose squares for sale on the street. Users ingest the LSD by swallowing or licking the squares or drinking a beverage into which the squares have been mixed.
In 1988, petitioner Meirl Neal was arrested for selling 11,456 doses of LSD on blotter paper. The combined weight of the LSD and the paper was 109.51 grams. Following a guilty plea in the United States District Court for the Central District of Illinois, petitioner was convicted of one count of possession of LSD with intent to distribute it, in violation of 21 U. S. C. § 841, and one count of conspiracy to possess LSD with intent to distribute it, in violation of 21 U. S. C. § 846. At the initial sentencing, the method for determining the weight of the illegal mixture or substance was the same under the Guidelines and the statute directing minimum sentences. The determinative amount was the whole weight of the blotter paper containing the drug. See United States Sentencing Commission, Guidelines Manual §2D1.1, Drug Quantity Table, n: * (Nov. 1987) (1987 USSG). Because the total weight of the LSD and blotter paper exceeded 10 grams, the District Court found petitioner subject to the 10-year mandatory minimum sentence specified in 21 U. S. C. § 841(b)(l)(A)(v). Under the version of the Sentencing Guidelines then in effect, the indicated sentence was even greater than the statutory minimum: The quantity of the *287 drugs and petitioner’s prior convictions resulted in a Guidelines sentencing range of 188 to 235 months’ imprisonment, even after an adjustment for petitioner’s acceptance of responsibility for the crime. The District Court imposed concurrent sentences of 192 months’ imprisonment on each count, to be followed by five years of supervised release.
In November 1993, the United States Sentencing Commission revised the method of calculating the weight of LSD in the Sentencing Guidelines. United States Sentencing Commission, Guidelines Manual, App. C., Arndt. 488 (Nov. 1995) (1995 USSG). Departing from its former approach of weighing the entire mixture or substance containing LSD, the amended Guideline instructed courts to give each dose of LSD on a carrier medium a constructive or presumed weight of 0.4 milligrams. Id., §2Dl.l(c), n. (H). The revised Guideline was retroactive, id., App. C, Arndt. 502, and one month later petitioner filed a motion to modify his sentence, see 18 U. S. C. § 3582(c)(2). He contended that the weight of the LSD attributable to him under the amended Guidelines was 4.58 grams (11,456 doses x 0.4 milligrams). For that amount, the applicable sentencing range under the Guidelines would be 70 to 87 months of imprisonment. The 10-year statutory minimum of § 841(b)(l)(A)(v) was no bar to a reduced sentence, petitioner argued, because the presumptive-weight method of the Guidelines should also control the mandatory minimum calculation. The 4.58 grams attributable to him by the Guidelines method would be well short of the 10 grams necessitating a 10-year minimum sentence.
The District Court, following our recent decision in
Chapman
v.
United States,
On appeal, the Court of Appeals for the Seventh Circuit, sitting en banc, agreed with the District Court that a dual system now prevails in calculating LSD weights in cases like this, and it affirmed petitioner’s sentence.
We granted certiorari to resolve a conflict in the Courts of Appeals over whether the revised Guideline governs the calculation of the weight of LSD for purposes of § 841(b)(1).
II
Congress has tried different punishment schemes to combat the menace of drug trafficking. Under the Comprehensive Drug Abuse Prevention and Control Act of 1970, Pub. L. 91-513, 84 Stat. 1236, penalties depended upon whether or not a drug was classified as a narcotic. Chagrined by the resulting sentencing disparities, Congress amended the narcotics laws in 1984 to calculate penalties by the weight of the pure drug involved. See
Chapman,
In
Chapman,
we interpreted the provision of the Act that provided a mandatory minimum sentence of five years for trafficking in an LSD “mixture or substance” that weighed one gram or more, see § 841(b)(l)(B)(v). We construed “mixture” and “substance” to have their ordinary meaning, observing that the terms had not been defined in the statute or the Sentencing Guidelines and had no distinctive common-law meaning.
Petitioner contends that the method approved in
Chapman
is no longer appropriate. In his view, the Commission intended its dose-based method to supplant the actual-weight method used in
Chapman,
and we should not presume that the Commission would recognize two inconsistent schemes for sentencing LSD traffickers, cf. 1995 USSG §2D1.1, comment., backg’d (stating purposes of making offense levels in §2D1.1 “proportional to the levels established
*290
by statute” and providing “a logical sentencing structure for drug offenses”). Petitioner concedes, as he must, that the Commission does not have the authority to amend the statute we construed in
Chapman.
He argues, nonetheless, that the Commission is the agency charged with interpretation of penalty statutes and expert in sentencing matters, so its construction of § 841(b)(1) should be given deference. See
Chevron U. S. A. Inc.
v.
Natural Resources Defense Council, Inc.,
While acknowledging that the Commission’s expertise and the design of the Guidelines may be of potential weight and relevance in other contexts, we conclude that the Commission’s choice of an alternative methodology for weighing LSD does not alter our interpretation of the statute in Chapman. In any event, principles of stare decisis require that we adhere to our earlier decision.
The Commission was born of congressional disenchantment with the vagaries of federal sentencing and of the parole system.
Mistretta
v.
United States,
Like 21 U. S. C. § 841(b)(1), the Sentencing Guidelines calibrate the punishment of drug traffickers according to the quantity of drugs involved in the offense. From their inception in 1987, the Guidelines have used a detailed Drug Quantity Table as a first step in determining the sentence. See 1995 USSG §2D1.1(c); 1987 USSG §2D1.1. The current Table has 17 tiers, each with specified weight ranges for different controlled substances. The weight ranges reflect the Commission’s assessment of equivalent culpability among defendants who traffic in different types of drugs, and so all defendants in the same tier are assigned the same base offense level. See 1995 USSG § 2D 1.1(c). Once the base offense level is adjusted for other factors, it yields a specific sentencing range depending on the defendant’s criminal history. See 1995 USSG § 1B1.1.
Although the mandatory-minimum statute and the Guidelines both turn upon drug quantities, the Commission itself has noted that “mandatory mínimums are both structurally and functionally at odds with sentencing guidelines and the goals the guidelines seek to achieve.” United States Sentencing Commission, Special Report to Congress: Mandatory Minimum Penalties in the Federal Criminal Justice System 26 (Aug. 1991).
*292 “The sentencing guidelines system is essentially a system of finely calibrated sentences. For example, as the quantity of drugs increases, there is a proportional increase in the sentence. In marked contrast, the mandatory mínimums are essentially a flat, tariff-like approach to sentencing. Whereas guidelines seek a smooth continuum, mandatory mínimums result in ‘cliffs.’ The ‘cliffs’ that result from mandatory mínimums compromise proportionality, a fundamental premise for just punishment, and a primary goal of the Sentencing Reform Act.” Id., at iii.
Despite incongruities between the Guidelines and the mandatory sentencing statute, the Commission has sought to make the Guidelines parallel to the scheme of § 841(b)(1) in most instances. See 1995 USSG §2D1.1, comment., n. 10 (“The Commission has used the sentences provided in, and equivalences derived from, the statute (21 U. S. C. § 841(b)(1)), as the primary basis for the guideline sentences”). As a general rule, the Commission adopts the same approach to weighing drugs as the statute does: “Unless otherwise specified, the' weight of a controlled substance set forth in the table refers to the entire weight of any mixture or substance containing a detectable amount of the controlled substance.” 1995 USSG §2D1.1(c), n. (A); see also 1995 USSG §2D1.1, comment., n. 1 (“‘Mixture or substance’ as used in this guideline has the same meaning as in 21 U. S. C. § 841, except as expressly provided”); 1987 USSG §2D1.1, n. * (weighing rule intended to be “[consistent with the provisions of the Anti-Drug Abuse Act”). For most narcotics, there will be no inconsistency in the calculations of drug quantities.
After study of the LSD trade, however, the Commission in 1993 decided it could no longer follow that general rule for LSD offenses and fulfill the statutory directive to promote proportionate sentencing. The Commission determined that “[bjecause the weights of LSD carrier media vary *293 widely and typically far exceed the weight of the controlled substance itself,... basing offense levels on the entire weight of the LSD and carrier medium would produce unwarranted disparity among offenses involving the same quantity of actual LSD (but different carrier weights), as well as sentences disproportionate to those for other, more dangerous controlled substances, such as PCP.” 1995 USSG §2D1.1, comment., backg’d. To remedy the problem, the Commission revised its Guideline to provide:
“In the case of LSD on a carrier medium (e. g., a sheet of blotter paper), do not use the weight of the LSD/carrier medium. Instead, treat each dose of LSD on the carrier medium as equal to 0.4 mg of LSD for the purposes of the Drug Quantity Table.” 1995 USSG §2Dl.l(c), n. (H).
Under the amendment, a court determines the defendant’s base offense level in the Drug Quantity Table by multiplying the number of doses of LSD involved by 0.4 milligrams. The Commission submitted the amendment to Congress, which did not disapprove it in the 180 days allotted by statute. See 28 U. S. C. § 994(p). The amended Guideline took effect on November 1,1993, and was in force when petitioner was resentenced.
As a threshold matter, it is doubtful that the Commission intended the constructive-weight method of the Guidelines to displace the actual-weight method that
Chapman
requires for statutory minimum sentences. The commentary, which is the authoritative construction of the Guidelines absent plain inconsistency or statutory or constitutional infirmity,
Stinson
v.
United States,
Furthermore, after a catalog of reasons for adopting the dose-based method, the Commission ends with the observation that “[Nonetheless, this approach does not override the applicability of ‘mixture or substance’ for the purpose of applying any mandatory minimum sentence (see Chapman; §5Gl.l(b)).” Ibid. The citation of Chapman in tandem with § 5G1.1(b), which advises that “[w]here a statutorily required minimum sentence is greater than the maximum of the applicable guideline range, the statutorily required minimum sentence shall be the guideline sentence,” suggests that the Guidelines calculation is independent of the statutory calculation, and that the statute controls if they conflict. The Commission seems to do no more than acknowledge that, whether or not its method would be preferable for the statute and Guideline alike, it has no authority to override the statute as we have construed it.
Were we, for argument’s sake, to adopt petitioner’s view that the Commission intended the commentary as an interpretation of § 841(b)(1), and that the last sentence of the commentary states the Commission’s view that the dose-based method is consistent with the term “mixture or substance” in the statute, he still would not prevail. The Commission’s dose-based method cannot be squared with
Chapman.
The Guideline does take into account some of the weight of the carrier medium (because the average weight of an LSD dose is 0.05 milligrams, 0.35 of the Commission’s constructive weight per dose of 0.4 milligrams is attributable to the medium, see 1995 USSG §2D1.1, comment., backg’d), but we
*295
held in
Chapman
that § 841(b)(1) requires “the entire mixture or substance ... to be weighed when calculating the sentence.”
Our reluctance to overturn precedents derives in part from institutional concerns about the relationship of the Judiciary to Congress. One reason that we give great weight to
stare decisis
in the area of statutory construction is that “Congress is free to change this Court’s interpretation of its legislation.”
Illinois Brick Co.
v.
Illinois,
Like
Chapman,
this case involves a petitioner who sold LSD on blotter paper, the “carrier of choice” involved in “the vast majority of cases.”
We hold that § 841(b)(1) directs a sentencing court to take into account the actual weight of the blotter paper with its absorbed LSD, even though the Sentencing Guidelines require a different method of calculating the weight of an LSD mixture or substance. The judgment of the Court of Appeals is affirmed.
It is so ordered.
