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                                                                    Exhibit 10.5

                                  SEPRACOR INC.

                        1998 EMPLOYEE STOCK PURCHASE PLAN

       The purpose of this Plan is to provide eligible employees of Sepracor
Inc. (the "Company") and certain of its subsidiaries with opportunities to
purchase shares of the Company's common stock, $.10 par value (the "Common
Stock"), commencing on June 1, 1998. Three hundred thousand (300,000) shares of
Common Stock in the aggregate have been approved for this purpose.

       1. ADMINISTRATION. The Plan will be administered by the Company's Board
of Directors (the "Board") or by a Committee appointed by the Board (the
"Committee"). The Board or the Committee has authority to make rules and
regulations for the administration of the Plan and its interpretation and
decisions with regard thereto shall be final and conclusive.

       2. ELIGIBILITY. Participation in the Plan will neither be permitted nor
denied contrary to the requirements of Section 423 of the Internal Revenue Code
of 1986, as amended (the "Code"), and regulations promulgated thereunder. All
employees of the Company, including Directors who are employees, and all
employees of any subsidiary of the Company (as defined in Section 424(f) of the
Code) designated by the Board or the Committee from time to time (a "Designated
Subsidiary"), are eligible to participate in any one or more of the offerings of
Options (as defined in Section 9) to purchase Common Stock under the Plan
provided that:

               a) they are customarily employed by the Company or a Designated
Subsidiary for more than 20 hours a week and for more than five months in a
calendar year; and

               b) they are employees of the Company or a Designated Subsidiary
on the first day of the applicable Plan Period (as defined below).

       No employee may be granted an option hereunder if such employee,
immediately after the option is granted, owns 5% or more of the total combined
voting power or value of the stock of the Company or any subsidiary. For
purposes of the preceding sentence, the attribution rules of Section 424(d) of
the Code shall apply in determining the stock ownership of an employee, and all
stock which the employee has a contractual right to purchase shall be treated as
stock owned by the employee.

       3. OFFERINGS. The Company will make one or more offerings ("Offerings")
to employees to purchase stock under this Plan. Offerings will begin each June 1
and December 1, or the first business day thereafter (the "Offering Commencement
Dates"). Each Offering Commencement Date will begin a six-month period (a "Plan
Period") during which payroll deductions will be made and held for the purchase
of Common Stock at the end of the Plan Period. The Board or the

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Committee may, at its discretion, choose a different Plan Period of twelve (12)
months or less for subsequent Offerings.

       4. PARTICIPATION. An employee eligible on the Offering Commencement Date
of any Offering may participate in such Offering by completing and forwarding a
payroll deduction authorization form to the employee's appropriate payroll
office at least 30 days prior to the applicable Offering Commencement Date. The
form will authorize a regular payroll deduction from the Compensation received
by the employee during the Plan Period. Unless an employee files a new form or
withdraws from the Plan, his deductions and purchases will continue at the same
rate for future Offerings under the Plan as long as the Plan remains in effect.
The term "Compensation" means the amount of money reportable on the employee's
Federal Income Tax Withholding Statement, excluding overtime, incentive or bonus
awards, allowances and reimbursements for expenses such as relocation allowances
for travel expenses, income or gains on the exercise of Company stock options,
and similar items, whether or not shown on the employee's Federal Income Tax
Withholding Statement.

       5. DEDUCTIONS. The Company will maintain payroll deduction accounts for
all participating employees. Payroll deductions may be at the rate of 1%, 2%,
3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10% of Compensation with any change in
compensation during the Plan Period to result in an automatic corresponding
change in the dollar amount withheld.

       No employee may be granted an Option (as defined in Section 9) which
permits his rights to purchase Common Stock under this Plan and any other
employee stock purchase plan (as defined in Section 423(b) of the Code) of the
Company and its subsidiaries, to accrue at a rate which exceeds $25,000 of the
fair market value of such Common Stock (determined at the Offering Commencement
Date of the Plan Period) for each calendar year in which the Option is
outstanding at any time.

       6. DEDUCTION CHANGES. An employee may decrease or discontinue his payroll
deduction once during any Plan Period, by filing a new payroll deduction
authorization form. However, an employee may not increase his payroll deduction
during a Plan Period. If an employee elects to discontinue his payroll
deductions during a Plan Period, but does not elect to withdraw his funds
pursuant to Section 8 hereof, funds deducted prior to his election to
discontinue will be applied to the purchase of Common Stock on the Exercise Date
(as defined below).

       7. INTEREST. Interest will not be paid on any employee accounts, except
to the extent that the Board or the Committee, in its sole discretion, elects to
credit employee accounts with interest at such per annum rate as it may from
time to time determine.

       8. WITHDRAWAL OF FUNDS. An employee may at any time prior to the close of
business on the last business day in a Plan Period and for any reason
permanently draw out the balance

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accumulated in the employee's account and thereby withdraw from participation in
an Offering. Partial withdrawals are not permitted. The employee may not begin
participation again during the remainder of the Plan Period. The employee may
participate in any subsequent Offering in accordance with terms and conditions
established by the Board or the Committee.

       9. PURCHASE OF SHARES. On the Offering Commencement Date of each Plan
Period, the Company will grant to each eligible employee who is then a
participant in the Plan an option ("Option") to purchase on the last business
day of such Plan Period (the "Exercise Date"), at the Option Price hereinafter
provided for, the largest number of whole shares of Common Stock of the Company
as does not exceed the number of shares determined by dividing the Plan Period
Limit by the closing price (as defined below) on the Offering Commencement Date
of such Plan Period. The Plan Period Limit shall be equal to $208.33 multiplied
by the number of whole months in the Plan Period, rounded to the nearest whole
dollar.

       The purchase price for each share purchased will be 85% of the closing
price of the Common Stock on (i) the first business day of such Plan Period or
(ii) the Exercise Date, whichever closing price shall be less. Such closing
price shall be (a) the closing price on any national securities exchange on
which the Common Stock is listed, (b) the closing price of the Common Stock on
the Nasdaq National Market or (c) the average of the closing bid and asked
prices in the over-the-counter-market, whichever is applicable, as published in
The Wall Street Journal. If no sales of Common Stock were made on such a day,
the price of the Common Stock for purposes of clauses (a) and (b) above shall be
the reported price for the next preceding day on which sales were made.

       Each employee who continues to be a participant in the Plan on the
Exercise Date shall be deemed to have exercised his Option at the Option Price
on such date and shall be deemed to have purchased from the Company the number
of full shares of Common Stock reserved for the purpose of the Plan that his
accumulated payroll deductions on such date will pay for, but not in excess of
the maximum number determined in the manner set forth above.

       Any balance remaining in an employee's payroll deduction account at the
end of a Plan Period will be automatically refunded to the employee, except that
any balance which is less than the purchase price of one share of Common Stock
will be carried forward into the employee's payroll deduction account for the
following Offering, unless the employee elects not to participate in the
following Offering under the Plan, in which case the balance in the employee's
account shall be refunded.

       10. ISSUANCE OF CERTIFICATES. Certificates representing shares of Common
Stock purchased under the Plan may be issued only in the name of the employee,
in the name of the employee and another person of legal age as joint tenants
with rights of survivorship, or (in the Company's sole discretion) in the name
of a brokerage firm, bank or other nominee holder designated by the employee.
The Company may, in its sole discretion and in compliance with applicable laws,
authorize the use of book entry registration of shares in lieu of issuing stock
certificates.

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       11. RIGHTS ON RETIREMENT, DEATH OR TERMINATION OF EMPLOYMENT. In the
event of a participating employee's termination of employment prior to the last
business day of a Plan Period, no payroll deduction shall be taken from any pay
due and owing to an employee and the balance in the employee's account shall be
paid to the employee or, in the event of the employee's death, (a) to a
beneficiary previously designated in a revocable notice signed by the employee
(with any spousal consent required under state law) or (b) in the absence of
such a designated beneficiary, to the executor or administrator of the
employee's estate or (c) if no such executor or administrator has been appointed
to the knowledge of the Company, to such other person(s) as the Company may, in
its discretion, designate. If, prior to the last business day of the Plan
Period, the Designated Subsidiary by which an employee is employed shall cease
to be a subsidiary of the Company, or if the employee is transferred to a
subsidiary of the Company that is not a Designated Subsidiary, the employee
shall be deemed to have terminated employment for the purposes of this Plan.

       12. OPTIONEES NOT STOCKHOLDERS. Neither the granting of an Option to an
employee nor the deductions from his pay shall constitute such employee a
stockholder of the shares of Common Stock covered by an Option under this Plan
until such shares have been purchased by and issued to him.

       13. RIGHTS NOT TRANSFERABLE. Rights under this Plan are not transferable
by a participating employee other than by will or the laws of descent and
distribution, and are exercisable during the employee's lifetime only by the
employee.

       14. APPLICATION OF FUNDS. All funds received or held by the Company under
this Plan may be combined with other corporate funds and may be used for any
corporate purpose.

       15. ADJUSTMENT IN CASE OF CHANGES AFFECTING COMMON STOCK. In the event of
a subdivision of outstanding shares of Common Stock, or the payment of a
dividend in Common Stock, the number of shares approved for this Plan, and the
share limitation set forth in Section 9, shall be increased proportionately, and
such other adjustment shall be made as may be deemed equitable by the Board or
the Committee. In the event of any other change affecting the Common Stock, such
adjustment shall be made as may be deemed equitable by the Board or the
Committee to give proper effect to such event.

       16. MERGER. If the Company shall at any time merge or consolidate with
another corporation and the holders of the capital stock of the Company
immediately prior to such merger or consolidation continue to hold at least 80%
by voting power of the capital stock of the surviving corporation ("Continuity
of Control"), the holder of each Option then outstanding will thereafter be
entitled to receive at the next Exercise Date upon the exercise of such Option
for each share as to which such Option shall be exercised the securities or
property which a holder of one share of the Common Stock was entitled to upon
and at the time of such merger or consolidation, and the Board or the Committee
shall take such steps in connection with such merger or consolidation as the
Board or the Committee shall deem necessary to assure that the

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provisions of Section 15 shall thereafter be applicable, as nearly as reasonably
may be, in relation to the said securities or property as to which such holder
of such Option might thereafter be entitled to receive thereunder.

       In the event of a merger or consolidation of the Company with or into
another corporation which does not involve Continuity of Control, or of a sale
of all or substantially all of the assets of the Company while unexercised
Options remain outstanding under the Plan, (a) subject to the provisions of
clauses (b) and (c), after the effective date of such transaction, each holder
of an outstanding Option shall be entitled, upon exercise of such Option, to
receive in lieu of shares of Common Stock, shares of such stock or other
securities as the holders of shares of Common Stock received pursuant to the
terms of such transaction; or (b) all outstanding Options may be cancelled by
the Board or the Committee as of a date prior to the effective date of any such
transaction and all payroll deductions shall be paid out to the participating
employees; or (c) all outstanding Options may be cancelled by the Board or the
Committee as of the effective date of any such transaction, provided that notice
of such cancellation shall be given to each holder of an Option, and each holder
of an Option shall have the right to exercise such Option in full based on
payroll deductions then credited to his account as of a date determined by the
Board or the Committee, which date shall not be less than ten (10) days
preceding the effective date of such transaction.

       17. AMENDMENT OF THE PLAN. The Board may at any time, and from time to
time, amend this Plan in any respect, except that (a) if the approval of any
such amendment by the shareholders of the Company is required by Section 423 of
the Code, such amendment shall not be effected without such approval, and (b) in
no event may any amendment be made which would cause the Plan to fail to comply
with Section 423 of the Code.

       18. INSUFFICIENT SHARES. In the event that the total number of shares of
Common Stock specified in elections to be purchased under any Offering plus the
number of shares purchased under previous Offerings under this Plan exceeds the
maximum number of shares issuable under this Plan, the Board or the Committee
will allot the shares then available on a pro rata basis.

       19. TERMINATION OF THE PLAN. This Plan may be terminated at any time by
the Board. Upon termination of this Plan all amounts in the accounts of
participating employees shall be promptly refunded.

       20. GOVERNMENTAL REGULATIONS. The Company's obligation to sell and
deliver Common Stock under this Plan is subject to listing on a national stock
exchange or quotation on the Nasdaq National Market and the approval of all
governmental authorities required in connection with the authorization, issuance
or sale of such stock.

       21. GOVERNING LAW. The Plan shall be governed by Delaware law except to
the extent that such law is preempted by federal law.

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       22. ISSUANCE OF SHARES. Shares may be issued upon exercise of an Option
from authorized but unissued Common Stock, from shares held in the treasury of
the Company, or from any other proper source.

       23. NOTIFICATION UPON SALE OF SHARES. Each employee agrees, by entering
the Plan, to promptly give the Company notice of any disposition of shares
purchased under the Plan where such disposition occurs within two years after
the date of grant of the Option pursuant to which such shares were purchased.

       24. Effective Date and Approval of Shareholders. The Plan shall take
effect on __________________, 1998 subject to approval by the shareholders of
the Company as required by Section 423 of the Code, which approval must occur
within twelve months of the adoption of the Plan by the Board.

                          Adopted by the Board of Directors on February 26, 1998

                                    Approved by the stockholders on May 27, 1998

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                                  SEPRACOR INC.

              AMENDMENT NO. 1 TO 1998 EMPLOYEE STOCK PURCHASE PLAN

The 1998 Employee Stock Purchase Plan (the "Plan") be and hereby is amended by
deleting the second sentence of the first paragraph of the Plan in its entirety
and inserting in lieu thereof the following:

       "Nine hundred thousand (900,000) shares of Common Stock in the aggregate
       have been approved for this purpose."

                         Adopted by the Board of Directors on February 20, 2003.

                                    Approved by the stockholders on May 22, 2003