<PAGE>

                                                                     EXHIBIT 4.6

                    [Xcel Pharmaceuticals, Inc. Letterhead]

March 30, 2001



VIA OVERNIGHT COURIER
---------------------

Elan International Services, Ltd.
102 St. James Court
Flatts, Smiths, FL04, Bermuda

         Re:   Election of Elan International Services, Ltd.'s nominee to Xcel
               Pharmaceuticals, Inc.'s Board of Directors

Ladies and Gentlemen:

Following expiration or termination of that certain Voting Agreement between
Xcel Pharmaceuticals, Inc. (the "Company") and the Series A Investors and Common
Holders identified therein dated March 30, 2001, the Company together with the
other signatories hereto (New Enterprise Associates, Domain Partners, V, L.P.
and DP V Associates, L.P. (collectively, the "Signatories")) agree that, at the
request of Elan International Services, Ltd. and/or one of its "Affiliates" (as
defined in Rule 405 promulgated under the Securities Act of 1933, as amended)
(collectively, "Elan"):

         1.    the Company and the Signatories will use their best efforts to
               nominate and recommend a nominee of Elan who is approved by the
               Company (which approval shall not be unreasonable withheld) (an
               "Elan Nominee") for election to the Company's Board of Directors
               (the "Board") at successive annual meetings of the stockholders
               of the Company; and

         2.    the Signatories will vote any and all shares of the Company's
               Common Stock then held by each Signatory to elect the Elan
               Nominee to the Board.

The Company's and the Signatories' agreement to nominate, recommend and vote
their respective shares of the Company's Common Stock to elect an Elan Nominee
shall terminate in the event that the Company is no longer indebted to Elan. If
an Elan Nominee is a member of the Board at such time as the Company is no
longer indebted to Elan, such Elan Nominee Board member shall tender his or her
resignation as a Board member within sixty (60) days after receipt of the
                                     ----- ----
Company's written request that he or she resign.

<PAGE>

March 30, 2001
Page 2



                           [Signature Page to Follow]

Very truly yours,

XCEL PHARMAEUTICALS, INC.                     NEW ENTERPRISE ASSOCIATES



By: /s/ Michael T. Borer                By: /s/ Thomas C. McConnell
   --------------------------------        -------------------------------------
Name: Michael T. Borer                  Name: Thomas C. McConnell
      -----------------------------           ----------------------------------
Its:  President                         Its:  General Partner
      -----------------------------           ----------------------------------


DOMAIN PARTNERS V, L.P.                       DP V ASSOCIATES, L.P.



By: /s/ Kathleen K. Schoemaker          By: /s/ Kathleen K. Schoemaker
   --------------------------------        -------------------------------------
Name: Kathleen K. Schoemaker            Name: Kathleen K. Schoemaker
      -----------------------------           ----------------------------------
Its:  Managing Member                   Its:  Managing Member
      -----------------------------           ----------------------------------



                            * * * * * * * * * * * * *



Acknowledged By:

ELAN INTERNATIONAL SERVICES, LTD.



By: /s/ Debra Moore
   --------------------------------
Name: Debra Moore
      -----------------------------
Its:  Vice President
      -----------------------------



  [Signature Page to Letter Re: Election of Elan International Services, Ltd.'s
          nominee to Xcel Pharmaceuticals, Inc.'s Board of Directors]