29 C.F.R. § 1630.3

Exceptions to the definitions of “Disability” and “Qualified Individual with a Disability.”

            (a) The terms
            disability
             and
            qualified individual with a disability
             do not include individuals currently engaging in the illegal use of drugs, when the covered entity acts on the basis of such use.

            (1)
            Drug
             means a controlled substance, as defined in schedules I through V of section 202 of the Controlled Substances Act (21 U.S.C 812)

            (2)
            Illegal use of drugs
             means the use of drugs the possession or distribution of which is unlawful under the Controlled Substances Act, as periodically updated by the Food and Drug Administration. This term does not include the use of a drug taken under the supervision of a licensed health care professional, or other uses authorized by the Controlled Substances Act or other provisions of Federal law.

            (b) However, the terms
            disability
             and
            qualified
             individual with a disability may not exclude an individual who:

        (1) Has successfully completed a supervised drug rehabilitation program and is no longer engaging in the illegal use of drugs, or has otherwise been rehabilitated successfully and is no longer engaging in the illegal use of drugs; or

            (2) Is participating in a supervised rehabilitation program and is no longer engaging in such use; or

        (3) Is erroneously regarded as engaging in such use, but is not engaging in such use.

        (c) It shall not be a violation of this part for a covered entity to adopt or administer reasonable policies or procedures, including but not limited to drug testing, designed to ensure that an individual described in paragraph (b) (1) or (2) of this section is no longer engaging in the illegal use of drugs. (See § 1630.16(c) Drug testing).

            (d)
            Disability
             does not include:

        (1) Transvestism, transsexualism, pedophilia, exhibitionism, voyeurism, gender identity disorders not resulting from physical impairments, or other sexual behavior disorders;

        (2) Compulsive gambling, kleptomania, or pyromania; or

        (3) Psychoactive substance use disorders resulting from current illegal use of drugs.

            (e)
            Homosexuality and bisexuality
             are not impairments and so are not disabilities as defined in this part.

Official source