Filed pursuant to Rule 424(b)(3)
Registration No. 333-89792

Prospectus Supplement No. 2 dated July 22, 2002
(To Prospectus dated July 9, 2002)

$1,204,950,000
IDEC Pharmaceuticals Corporation
Liquid Yield Option™ Notes due 2032
(Zero Coupon—Senior)
and Shares of Common Stock Issuable Upon Conversion and/or Purchase
of the Liquid Yield Option™ Notes due 2032

        This document supplements the prospectus dated July 9, 2002, as amended and supplemented through the date hereof, relating to the resale by the holders of Liquid Yield Option™ Notes (Zero Coupon—Senior) (the "LYONs") due April 29, 2032 and the shares of our common stock into which the LYONs are convertible.

        This prospectus supplement is incorporated by reference into, and should be read in conjunction with, the prospectus dated July 9, 2002, including any amendments or supplements to it. This prospectus supplement is not complete without, and may not be delivered or utilized except in connection with, the prospectus, including any amendments or supplements to it.

        The table presented below sets forth information as of the date of this prospectus supplement about the principal amount at maturity of the LYONs and the underlying common stock beneficially owned by each selling security holder that may be offered using the prospectus. All of the information set forth in the table has been provided by the selling security holders on or prior to the date of this prospectus supplement and, to our knowledge, is true and correct as of the date of this prospectus supplement. However, the selling security holders may have sold, transferred or otherwise disposed of all or a portion of their LYONs since the date on which they provided the information regarding their LYONs.

        The table of selling security holders contained on pages 50-53 of the prospectus dated July 9, 2002, as amended and supplemented through the date hereof, is hereby amended by and replaced in its entirety with the table set forth below.

Name

  Aggregate Principal
Amount at Maturity of
LYONs that May Be Sold

  Percentage of LYONs
Outstanding

  Number of Shares of
Common Stock that
May be Sold

  Percentage of
Common Stock
Outstanding

 
Advent Convertible Master Cayman L.P.   $ 13,542,000   1.12   97,341   *  
Allstate Insurance Company   $ 1,000,000   *   7,188   *  
Allstate Life Insurance Company   $ 500,000   *   3,594   *  
Alpha US Sub Fund 4, LLC   $ 1,508,000   *   10,839   *  
Argent Classic Convertible Arbitrage Fund (Bermuda) Ltd.   $ 8,000,000   *   57,504   *  
Argent Classic Convertible Arbitrage Fund L.P.   $ 4,700,000   *   33,784   *  
Argent LowLev Convertible Arbitrage Fund Ltd.   $ 9,000,000   *   64,693   *  
Argern LowLev Convertible Arbitrage Fund LLC   $ 800,000   *   5,750   *  
Bank Austria Cayman Islands, Ltd.   $ 3,500,000   *   25,158   *  
Bank of America Pension Plan   $ 7,000,000   *   50,316   *  
Black Diamond Capital I, Ltd.   $ 480,000   *   3,450   *  
Black Diamond Convertible Offshore LDC   $ 3,195,000   *   22,966   *  

Black Diamond Offshore Ltd.   $ 1,920,000   *   13,801   *  
CG Investments, LDC   $ 4,000,000   *   28,752   *  
Clinton Convertible Managed Trading Account 1   $ 6,800,000   *   48,879   *  
Clinton Multistrategy Master Fund, Ltd.   $ 16,600,000   1.38   16,607   *  
Clinton Riverside Convertible Portfolio Limited   $ 33,850,000   2.8   243,317   *  
Credit Suisse First Boston London Branch   $ 13,000,000   1.08   93,445   *  
Deutsche Bank Securities Inc.   $ 123,340,000   10.24   886,580   *  
Double Black Diamond Offshore LDC   $ 8,925,000   *   64,154   *  
Gaia Offshore Master Fund Ltd.   $ 4,300,000   *   30,908   *  
Global Bermuda Limited Partnership   $ 4,400,000   *   31,628   *  
HFR Convertible Arbitrage Account   $ 1,251,000   *   8,992   *  
JMG Convertible Investments, LP   $ 16,500,000   1.37   118,604   *  
JMG Triton Offshore Fund, Ltd.   $ 10,500,000   *   75,475   *  
KBC Financial Products (Cayman Islands) Limited   $ 100,000,000   8.3   718,810   *  
KBC Financial Products USA Inc.   $ 8,950,000   *   64,333   *  
Lakeshore International, Ltd.   $ 17,600,000   1.46   126,511   *  
LDG Limited   $ 817,430   *   5,875   *  
Lyxor   $ 3,000,000   *   21,564   *  
Lyxor Master Fund   $ 700,000   *   5,031   *  
Lyxor Master Fund Ref: Argent/LowLev CB   $ 2,500,000   *   17,970   *  
Man Convertible Bond Master Fund, Ltd.   $ 12,880,000   1.07   92,582   *  
Marathon Global Convertible Master Fund, Ltd.   $ 38,000,000   3.15   273,147   *  
McMahan Securities Co., L.P.   $ 3,000,000   *   21,564   *  
Minnesota Power and Light   $ 363,000   *   2,609   *  
Morgan Stanley & Co.   $ 15,000,000   1.25   107,821   *  
People's Benefit Life Insurance Company   $ 20,000,000   1.66   143,762   *  
Ram Trading LTD   $ 5,500,000   *   39,534   *  
RCG Halifax Master Fund, Ltd.   $ 3,000,000   *   21,543   *  
RCG Latitude Master Fund, Ltd.   $ 6,000,000   *   43,128   *  
RCG Multi-Strategy, LP   $ 12,500,000   1.04   89,851   *  
Royal Bank of Canada   $ 12,000,000   *   86,257   *  
Salomon Smith Barney Inc.   $ 10,000,000   *   71,881   *  
St. Albans Partners Ltd.   $ 7,000,000   *   50,316   *  
St. Thomas Trading, Ltd.   $ 22,120,000   1.84   159,000   *  
Tag Associates   $ 336,000   *   2,415   *  
TD Securities (USA) Inc.   $ 3,000,000   *   21,543   *  
TQA Master Fund, Ltd.   $ 15,495,670   1.29   111,384   *  

TQA Master Plus Fund, Ltd.   $ 4,935,660   *   35,478   *  
Tribeca Investments, LLC   $ 2,000,000   *   14,376   *  
UBS AG London Branch   $ 81,660,000   6.78   586,980   *  
UBS O'Connor LLC f/b/o UBS Global Equity Arbitrage Master Fund Ltd.   $ 10,000,000   *   71,881   *  
UBS Warburg, LLC   $ 11,400,000   *   81,944   *  
UFJ Investments Asia Limited   $ 8,000,000   *   57,504   *  
Wachovia Securities, Inc.   $ 12,250,000   1.02   88,054   *  
Wachovia Securities International LTD   $ 15,000,000   1.25   107,821   *  
Wilmington Trust Company as Owner/Trustee for the Forrestal Funding Master Trust   $ 85,000,000   7.05   610,989   *  
Worldwide Transactions Ltd.   $ 480,000   *   3,450   *  
Zurich Institutional Benchmark Master Fund Ltd.   $ 1,200,000       8,625   *  
All other holders of LYONs or future transferees, pledgees, donees, assignees or sucessors of any holders   $ 354,651,240   29.43 % 2,652,043      
Totals   $ 1,204,950,000   100.00 % 8,661,301   5.68 %

        See "Risk Factors" section beginning on page 10 of the prospectus dated July 9, 2002, as amended and supplemented through the date hereof, to read about factors you should consider before purchasing the LYONs or our common stock.

        Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of the prospectus dated July 9, 2002 or any amendment or supplement thereto. Any representation to the contrary is a criminal offense.

        The date of this prospectus supplement is July 22, 2002.

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