QuickLinks -- Click here to rapidly navigate through this document

Exhibit 32.1

CERTIFICATION PURSUANT TO 18 U.S.C. SECTION 1350,
AS ADOPTED PURSUANT TO
SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

        In connection with the Quarterly Report on Form 10-Q of Sepracor Inc. (the "Company") for the period ended June 30, 2009 as filed with the Securities and Exchange Commission on the date hereof (the "Report"), the undersigned, Adrian Adams, Chief Executive Officer of the Company, hereby certifies, pursuant to 18 U.S.C. Section 1350, that to his knowledge:

(1)

the Report fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and



(2)

the information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Company.

Date: August 5, 2009 /s/ ADRIAN ADAMS
Adrian Adams
Chief Executive Officer

        A signed original of this written statement required by Section 906 has been provided to Sepracor Inc. and will be retained by Sepracor Inc. and furnished to the Securities and Exchange Commission or its staff upon request.

QuickLinks

Exhibit 32.1