As of January 1, 2013, Medicare Part D prescription drug plans must provide coverage of Barbiturates "when used to treat epilepsy, cancer, or a chronic mental health disorder" and Benzodiazepines. As noted in Section 175 of the Medicare Improvements for Patients and Providers Act of 2008 (MIPPA):
SEC. 175. INCLUSION OF BARBITURATES AND BENZODIAZEPINES AS COVERED PART D DRUGS.
(a) In general. — Section 1860D-2(e)(2)(A) of the Social Security Act (42 U.S.C. 1395w-102(e)(2)(A)) is amended by inserting after "agents)," the following: "other than subparagraph (I) of such section (relating to barbiturates) if the barbiturate is used in the treatment of epilepsy, cancer, or a chronic mental health disorder, and other than subparagraph (J) of such section (relating to benzodiazepines),". 42 USC 1395w—102(e)(2)(A) (P.L. 110-275, §175(a))
(b) Effective date. The amendments made by subsection (a) shall apply to prescriptions dispensed on or after January 1, 2013.
As a note, previous to this 2013 change, Section 1860D-2(e)(2)(A) of the Social Security Act stated:
(e) Covered Part D Drug Defined.— ...
(A) In general.— Such term does not include drugs or classes of drugs, or their medical uses, which may be excluded from coverage or otherwise restricted under section 1927(d)(2), other than subparagraph (E) of such section (relating to smoking cessation agents), or under section 1927(d)(3), as such sections were in effect on the date of the enactment of this part. Such term also does not include a drug when used for the treatment of sexual or erectile dysfunction, unless such drug were used to treat a condition, other than sexual or erectile dysfunction, for which the drug has been approved by the Food and Drug Administration.
(B) Medicare covered drugs.— A drug prescribed for a part D eligible individual that would otherwise be a covered part D drug under this part shall not be so considered if payment for such drug as so prescribed and dispensed or administered with respect to that individual is available (or would be available but for the application of a deductible) under part A or B for that individual.