section 709(c) of the Office of National Drug Control Policy Reauthorization Act of 1998 ( 21 U.S.C. 1708(c) ). Such term shall include any such drug for any taxable year if such designation is in effect under such section at any time during such taxable year. (C) Fentanyl-related substance The term fentanyl-related substance means any substance that is structurally related to fentanyl by 1 or more of the following modifications: (i) By replacement of the phenyl portion of the phenethyl group by any monocycle, whether or not further substituted in or on the monocycle. (ii) By substitution in or on the phenethyl group with alkyl, alkenyl, alkoxyl, hydroxyl, halo, haloalkyl, amino, or nitro groups. (iii) By substitution in or on the piperidine ring with alkyl, alkenyl, alkoxyl, ester, ether, hydroxyl, halo, haloalkyl, amino, or nitro groups. (iv) By replacement of the aniline ring with any aromatic monocycle whether or not further substituted in or on the aromatic monocycle. (v) By replacement of the N–propionyl group with another acyl group. . (c) Effective date The amendments made by this section shall apply to taxable years beginning after the date of the enactment of this Act. (d) GAO report During the 60-day period beginning on the date which 10 years after the date of the enactment of this Act, the Comptroller General shall submit a written report to Congress regarding the amount of tax credits allowed under section 41(a)(4) of the Internal Revenue Code of 1986 for qualified drug threat mitigation expenses and the types of qualified drug threat mitigation research with respect to which such credits were allowed. The Comptroller General shall ensure that the data in such report is anonymous and that any studies undertaken to prepare such report do not impede qualified drug threat mitigation research.