Exhibit 24.2

                                POWER OF ATTORNEY

        KNOW BY ALL THESE PRESENTS, that the undersigned hereby constitutes and
appoints each of (i) Teresa Bair, Vice President, Corporate Development & Legal
Affairs of Athenex, Inc. (the "Company") and (ii) J. Nicholas Riehle, the
Company's Chief Financial Officer, as the undersigned's true and lawful
attorney-in-fact to:

    1.  prepare, execute in the undersigned's name and on the undersigned's
        behalf, and submit to the U.S. Securities and Exchange Commission (the
        "SEC") a Form ID, including amendments thereto, and any other documents
        necessary or appropriate to obtain codes and passwords enabling the
        undersigned to make electronic filings with the SEC of reports required
        by Section 16(a) of the Securities Exchange Act of 1934 (the "Exchange
        Act") or any rule or regulation of the SEC;

    2.  execute for and on behalf of the undersigned, in the undersigned's
        capacity as an officer, trustee and/or shareholder of the Company, Forms
        3, 4 and 5 in accordance with Section 16(a) of the Exchange Act and the
        rules and regulations thereunder, and any other forms or reports the
        undersigned may be required to file in connection with the undersigned's
        ownership, acquisition or disposition of securities of the Company;

    3.  do and perform any and all acts for and on behalf of the undersigned
        that may be necessary or desirable to complete and execute any such Form
        3, 4 or 5, or other form or report, and timely file such form or report
        with the SEC and any stock exchange or similar authority; and

    4.  take any other action of any type whatsoever in connection with the
        foregoing that, in the opinion of such attorney-in-fact, may be of
        benefit to, in the best interest of, or legally required by the
        undersigned, it being understood that the documents executed by such
        attorney-in-fact on behalf of the undersigned pursuant to this Power of
        Attorney shall be in such form and shall contain such terms and
        conditions as such attorney-in-fact may approve in his or her
        discretion.

    The undersigned hereby grants to such attorney-in-fact full power and
authority to do and perform any and every act and thing whatsoever requisite,
necessary or proper to be done in the exercise of any of the rights and powers
herein granted, as fully to all intents and purposes as the undersigned might or
could do if personally present, with full power of substitution or revocation,
hereby ratifying and confirming all that such attorney-in-fact, or his or her
substitute or substitutes, shall lawfully do or cause to be done by virtue of
this Power of Attorney and the rights and powers herein granted. The undersigned
acknowledges that the foregoing attorneys-in-fact, in serving in such capacity
at the request of the undersigned, are not assuming any of the undersigned's
responsibilities to comply with Section 16 of the Exchange Act.

    This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file Forms 3, 4 and 5 with respect to the
undersigned's holdings of and transactions in securities issued by the Company,
unless earlier revoked by the undersigned in a signed writing delivered to the
foregoing attorneys-in-fact. This Power of Attorney may be filed with the SEC as
a confirming statement of the authority granted herein.

    This Power of Attorney does not relieve the undersigned from responsibility
for compliance with the undersigned's obligations under the Exchange Act,
including, without limitation, the reporting requirements under Section 16 of
the Exchange Act. Additionally, although pursuant to this Power of Attorney the
Company will use commercially reasonable best efforts to timely and accurately
file Section 16 reports on behalf of the undersigned, the Company does not
represent or warrant that it will be able to in all cases timely and accurately
file Section 16 reports on behalf of the undersigned due to various factors and
the undersigned and the Company's need to rely on others for information,
including the undersigned and brokers of the undersigned.

                            [Signature page follows]

    IN WITNESS WHEREOF the undersigned has caused this Power of Attorney to be
executed as of this 8th day of June, 2017.

                                        /s/ MUI BING HOW TAMMY
                                        ---------------------------------------
                                        Name: MUI BING HOW TAMMY