21 U.S. Code § 844 – Penalties for simple possession
(a) Unlawful acts; penalties
It shall be unlawful for any person knowingly or intentionally to possess a controlled substance unless such substance was obtained directly, or pursuant to a valid prescription or order, from a practitioner, while acting in the course of his professional practice, or except as otherwise authorized by this subchapter or subchapter II of this chapter. It shall be unlawful for any person knowingly or intentionally to possess any list I chemical obtained pursuant to or under authority of a registration issued to that person under section 823 of this title or section 958 of this title if that registration has been revoked or suspended, if that registration has expired, or if the registrant has ceased to do business in the manner contemplated by his registration. It shall be unlawful for any person to knowingly or intentionally purchase at retail during a 30 day period more than 9 grams of ephedrine base, pseudoephedrine base, or phenylpropanolamine base in a scheduled listed chemical product, except that, of such 9 grams, not more than 7.5 grams may be imported by means of shipping through any private or commercial carrier or the Postal Service. Any person who violates this subsection may be sentenced to a term of imprisonment of not more than 1 year, and shall be fined a minimum of $1,000, or both, except that if he commits such offense after a prior conviction under this subchapter or subchapter II of this chapter, or a prior conviction for any drug, narcotic, or chemical offense chargeable under the law of any State, has become final, he shall be sentenced to a term of imprisonment for not less than 15 days but not more than 2 years, and shall be fined a minimum of $2,500, except, further, that if he commits such offense after two or more prior convictions under this subchapter or subchapter II of this chapter, or two or more prior convictions for any drug, narcotic, or chemical offense chargeable under the law of any State, or a combination of two or more such offenses have become final, he shall be sentenced to a term of imprisonment for not less than 90 days but not more than 3 years, and shall be fined a minimum of $5,000. Notwithstanding any penalty provided in this subsection, any person convicted under this subsection for the possession of flunitrazepam shall be imprisoned for not more than 3 years, shall be fined as otherwise provided in this section, or both. The imposition or execution of a minimum sentence required to be imposed under this subsection shall not be suspended or deferred. Further, upon conviction, a person who violates this subsection shall be fined the reasonable costs of the investigation and prosecution of the offense, including the costs of prosecution of an offense as defined in sections 1918 and 1920 of title 28, except that this sentence shall not apply and a fine under this section need not be imposed if the court determines under the provision of title 18 that the defendant lacks the ability to pay.
18 U.S. Code § 3607 – Special probation and expungement procedures for drug possessors
(a)Pre-judgment Probation.—If a person found guilty of an offense described in section 404 of the Controlled Substances Act (21 U.S.C. 844)—
(1) has not, prior to the commission of such offense, been convicted of violating a Federal or State law relating to controlled substances; and
(2) has not previously been the subject of a disposition under this subsection;
the court may, with the consent of such person, place him on probation for a term of not more than one year without entering a judgment of conviction. At any time before the expiration of the term of probation, if the person has not violated a condition of his probation, the court may, without entering a judgment of conviction, dismiss the proceedings against the person and discharge him from probation. At the expiration of the term of probation, if the person has not violated a condition of his probation, the court shall, without entering a judgment of conviction, dismiss the proceedings against the person and discharge him from probation. If the person violates a condition of his probation, the court shall proceed in accordance with the provisions of section 3565.
(b)Record of Disposition.—
A nonpublic record of a disposition under subsection (a), or a conviction that is the subject of an expungement order under subsection (c), shall be retained by the Department of Justice solely for the purpose of use by the courts in determining in any subsequent proceeding whether a person qualifies for the disposition provided in subsection (a) or the expungement provided in subsection (c). A disposition under subsection (a), or a conviction that is the subject of an expungement order under subsection (c), shall not be considered a conviction for the purpose of a disqualification or a disability imposed by law upon conviction of a crime, or for any other purpose.
18 U.S. Code § 3565 – Revocation of probation
(a)Continuation or Revocation.—If the defendant violates a condition of probation at any time prior to the expiration or termination of the term of probation, the court may, after a hearing pursuant to Rule 32.1 of the Federal Rules of Criminal Procedure, and after considering the factors set forth in section 3553(a) to the extent that they are applicable—
(1) continue him on probation, with or without extending the term or modifying or enlarging the conditions; or
(2) revoke the sentence of probation and resentence the defendant under subchapter A.
(b)Mandatory Revocation for Possession of Controlled Substance or Firearm or Refusal To Comply With Drug Testing.—If the defendant—
(1) possesses a controlled substance in violation of the condition set forth in section 3563(a)(3);
(2) possesses a firearm, as such term is defined in section 921 of this title, in violation of Federal law, or otherwise violates a condition of probation prohibiting the defendant from possessing a firearm;
(3) refuses to comply with drug testing, thereby violating the condition imposed by section 3563(a)(4);  or
(4) as a part of drug testing, tests positive for illegal controlled substances more than 3 times over the course of 1 year;
the court shall revoke the sentence of probation and resentence the defendant under subchapter A to a sentence that includes a term of imprisonment.
The power of the court to revoke a sentence of probation for violation of a condition of probation, and to impose another sentence, extends beyond the expiration of the term of probation for any period reasonably necessary for the adjudication of matters arising before its expiration if, prior to its expiration, a warrant or summons has been issued on the basis of an allegation of such a violation.