<PAGE>
 
                                  FORM 10-Q/A

                      SECURITIES AND EXCHANGE COMMISSION
                             Washington, DC 20549


                  QUARTERLY REPORT UNDER SECTION 13 OR 15(d)
                    OF THE SECURITIES EXCHANGE ACT OF 1934


For the Quarter Ended                        Commission File Number:  1-10432
June 30, 1998


                      ROBERTS PHARMACEUTICAL CORPORATION
            (Exact name of registrant as specified in its charter)


    NEW JERSEY                                               22-2429994
(State or other jurisdiction of                           (I.R.S. Employer
incorporation or organization)                            Identification No.)


  MERIDIAN CENTER II
  4 INDUSTRIAL WAY WEST
  EATONTOWN, NEW JERSEY                                       07724
(Address of principal executive offices)                    (Zip Code)


              Registrant's telephone number, including area code:
                                (732) 676-1200


Indicate by check mark whether the registrant (1) has filed all reports
required to be filed by section 13 or 15(d) of the Securities Exchange Act of
1934 during the preceding 12 months (or for such shorter period that the
registrant was required to file such reports), and (2) has been subject to
such filing requirements for the past 90 days.            
                                                   Yes  X    No     


              Class                              Outstanding Shares at
                                                 October 26, 1998
          Common Stock                           31,419,012
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                      ROBERTS PHARMACEUTICAL CORPORATION

                                     INDEX

                                                          Page


Part I

          Item 1 - Financial Statements                     2


Signatures                                                  10
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                      ROBERTS PHARMACEUTICAL CORPORATION
                          CONSOLIDATED BALANCE SHEETS
                                (in thousands)
                                  (Unaudited)

        
                                      June 30, 1998      December 31, 1997
                                      -------------      -----------------
ASSETS:

  Current assets:
     Cash and cash equivalents             $ 14,808             $ 42,950
     Marketable securities                   46,508               39,887
     Accounts receivable, net                38,041               24,730
     Inventory                               25,619               19,826
     Notes receivable, current               15,923                  225
     Deferred tax assets                      4,962                4,962 
     Net assets held for sale                     0                3,760
     Other current assets                     2,608                1,647
                                          ---------            ---------
     Total current assets                   148,469              137,987

  Fixed assets, net                          31,505               25,913
  Intangible assets                         324,893              190,724
  Notes receivable                              584                  729
  Deferred non-current tax asset             12,332               12,332
  Other assets                                  194                  170
                                          ---------            ---------
     Total assets                          $517,977             $367,855
                                          =========            =========









   The accompanying notes are an integral part of these financial statements.

                                     - 2 -
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                      ROBERTS PHARMACEUTICAL CORPORATION
                          CONSOLIDATED BALANCE SHEETS
                                (in thousands)
                                  (Unaudited)


                                       June 30, 1998      December 31, 1997
                                       -------------      -----------------
LIABILITIES AND
  SHAREHOLDERS' EQUITY

  Current liabilities:
     Current installments of 
       long-term debt                      $  9,235             $  8,037
     Accounts payable                        17,584               13,188
     Income taxes payable                     7,108                3,022
     Dividends payable                            0                  150
     Other current liabilities               21,942               15,584
                                          ---------            ---------
     Total current liabilities               55,869               39,981

  Long-term debt, excluding 
     current installments                   130,302               10,327
  Deferred income                             1,949                    0
  Other liabilities                               0                  244

  Shareholders' equity:
     Class B preferred stock,
     $.10 par 10,000,000 shares
     authorized, 808,822 and
     2,721,030 outstanding                        0                   48
  
     Common stock, $.01 par,
     100,000,000 shares authorized,
     27,883,309 and 22,961,707
     outstanding                                343                  299

     Additional paid-in capital             379,909              372,384
     Cumulative translation adjustments      (1,714)              (1,250)
     Retained earnings (deficit)            (48,444)             (53,941)
     Treasury stock, 387,594 shares
     of common stock, at cost                  (237)                (237)
                                          ---------            ---------
     Total shareholders' equity             329,857              317,303 
                                          ---------            ---------
     Total liabilities and
       shareholders' equity               $ 517,977            $ 367,855 
                                          =========            =========





   The accompanying notes are an integral part of these financial statements.

                                      - 3 -
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                      ROBERTS PHARMACEUTICAL CORPORATION
                     CONSOLIDATED STATEMENTS OF OPERATIONS
                     (In thousands except per share data)
                                  (Unaudited)


                                     For the six months    For the three months
                                        ended June 30,        ended June 30,
                                       1998       1997       1998       1997 
                                     -------    -------    -------   --------
Sales and Revenue:
 Sales                               $76,384    $56,616    $43,796    $30,286
 Other revenue                           827          0        568          0 
                                     -------    -------    -------    -------
Total sales and revenue               77,211     56,616     44,364     30,286 
                                     -------    -------    -------    -------
Operating costs and expenses:
 Cost of sales                        28,261     24,633     15,658     12,981
 Research & Development                6,168      3,657      3,508      1,823
 Marketing                            17,378     15,966      9,515      9,029
 Administration                       18,085     11,096     10,898      5,612 
                                     -------    -------    -------    -------
Total operating costs & expenses      69,892     55,352     39,579     29,445 
                                     -------    -------    -------    -------
Operating income                       7,319      1,264      4,785        841 
                                     -------    -------    -------    -------
Other income (expense):
 Interest income                       2,386      2,709      1,113      1,541
 Interest expense                       (466)      (432)      (197)      (178)
 Foreign currency gain (loss)            (25)       (64)         1        (51)
 Other income(expense), net              (99)       (48)       (56)       (48)
                                     -------    -------    -------    -------
Total other income                     1,796      2,165        861      1,264 
                                     -------    -------    -------    -------
Income from continuing operations
 before income taxes                   9,115      3,429      5,646      2,105

Provision for income taxes             3,584      1,231      2,260        817 
                                     -------    -------    -------    -------
Net income                           $ 5,531    $ 2,198    $ 3,386    $ 1,288 
                                     =======    =======    =======    =======
Net income per share of common stock, 
 basic and diluted:                  $  0.18    $  0.06    $  0.11    $  0.04 
                                     =======    =======    =======    =======
Weighted average number of common 
 shares outstanding:
 Basic                            30,134,518 27,883,309 31,143,362 27,613,736
 Diluted                          30,342,548 28,885,892 31,351,392 28,616,319











   The accompanying notes are an integral part of these financial statements.

                                      - 4 -
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                      ROBERTS PHARMACEUTICAL CORPORATION 
               CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME 
                                  (Unaudited)


                                 For the six months     For the three months
                                    ended June 30,          ended June 30,
                                   1998         1997       1998         1997 
                              ---------    ---------  ---------    ---------
Net income                     $  5,531    $   2,858   $  3,386    $   1,288 
                              ---------    ---------  ---------    ---------
Other comprehensive income:

 Foreign currency translation 
   adjustment                      (464)        (173)      (667)          67 
                              ---------    ---------  ---------    ---------
Other comprehensive income         (464)        (173)      (667)          67 
                              ---------    ---------  ---------    ---------
Comprehensive income           $  5,067    $   2,685   $  2,719    $   1,355  
                              =========    =========  =========    =========















   The accompanying notes are an integral part of these financial statements.

                                      - 5 -
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                      ROBERTS PHARMACEUTICAL CORPORATION
                     CONSOLIDATED STATEMENTS OF CASH FLOWS
                                (In thousands)
                                  (Unaudited)

                                                     For the six months
                                                        ended June 30,
                                                    1998               1997
                                                 ---------        ---------
Cash flows provided by operating activities:       $11,137          $ 4,479  
                                                 ---------        ---------
Cash flows from investing activities:
   (Purchase) redemption of marketable securities   (6,620)          (1,536)
   Purchases of intangible assets                 (143,654)          (2,990) 
   Purchases of fixed assets                        (6,392)            (382)  
   Collection of notes receivable                      171              610   
       Net cash provided by (used in)            ---------        ---------
          investing activities                    (156,495)          (4,298)  
                                                 ---------        ---------
Cash flows from financing activities:
   Debt financing                                  125,000             ---
   Payments on notes payable and 
     long term debt                                (15,291)          (5,795)  
   Net proceeds from issuance of
     common stock                                    3,029              828   
   Net proceeds from issuance of
     5% Preferred stock                              4,492            1,000   
   5% Preferred stock dividends paid                   (34)          (1,229)  
       Net cash used in                          ---------        ---------
          financing activities                     117,196           (5,196)  
                                                 ---------        ---------
Exchange rate changes on cash and
   cash equivalents                                     20              (24)  
                                                 ---------        ---------
Change in cash and cash equivalents                (28,142)          (5,039)  
Beginning cash and cash equivalents                 42,950           87,125   
                                                 ---------        ---------
Ending cash and cash equivalents                   $14,808          $82,086   
                                                 =========        =========
Supplemental cash flow information:
   Interest paid                                  $    729          $   934   
   Income taxes paid                              $     11          $     8   








   The accompanying notes are an integral part of these financial statements.

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1.   Summary of Significant Accounting Policies

Basis of Presentation

In the opinion of management, the accompanying consolidated financial statements
include all necessary adjustments, consisting of normal adjustments, required
for a fair presentation of results for the period reported. All dollar amounts
are presented in thousands, except per share data.

New Accounting Pronouncement

Statement of Financial Accounting Standards No. 131, "Disclosures about Segments
of an Enterprise Related Information" (SFAS No. 131), establishes standards for
the way that public business companies report information about operating
segments in annual financial statements and requires that those companies report
selected information about operating segments in annual financial statements and
requires that those companies report selected information about operating
segments in interim financial reports issued to stockholders. It also
establishes standards for related disclosures about products and services,
geographic areas, and major customers. This Statement supersedes SFAS No. 14,
"Financial Reporting for Segments of a Business Enterprise," but retains the
requirement to report information about major customers.

SFAS No. 131 is effective for financial statements for periods beginning after
December 15, 1997. In the initial year of application, comparative information
for earlier years is to be restated.

The adoption of this Statement will not have an impact on the Company's
consolidated results of operation, financial position or cash flow for the year
ended December 31, 1998.

Reclassification

Certain items have been reclassified to conform to the current year
presentation. These items are not material.

2.   Inventory

Inventory at June 30, 1998 consists of:

          Raw Materials       $ 4,528
          Work in Progress      1,087
          Finished Goods       20,004
                             --------
          Total               $25,619
                             ========


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3.   Change in Accounting Estimate

During the first quarter 1998, management made a change in accounting estimate
in the amount of $1.0 million relating to the accrual of rebates for a product
to which Roberts has sole United States distribution rights. The accrual had
been established, over a three year period, for rebates which, it was determined
in the first quarter, will not be paid by Roberts and therefore was required to
be reversed.

5.   Subsequent Event

The Company made an investment of $10 million in the convertible preferred stock
of RiboGene, Inc., a drug discovery company targeting infectious diseases. The
shares have no voting rights. The investment will be carried on the cost method,
and none of the operating results of RiboGene will be included in Roberts'
income statement. One-third of the preferred stock is convertible to common
stock of RiboGene at each of the first three anniversary dates of the investment
at a conversion price of $7.00.

The Company also entered into an arrangement whereby Roberts will develop a new
delivery formulation for RiboGene's product EMITASOL. Under the terms of the
agreement, RiboGene will provide up to $7 million in funding from the
development of EMITASOL through completion of Phase III trials and the
submission of a New Drug Application ("NDA") with the balance, if any, provided
by Roberts. Upon approval of the NDA, Roberts can exercise its option to market
EMITASOL in the United States, Canada and Mexico under the RiboGene patents by
making a milestone payment at that time plus subsequent royalties on product
sales.

6.   Sale of Discontinued Division

During the second quarter 1998, the Company sold the discontinued VRG division
and a note was taken back for $9.7 million. As the first scheduled payment owed
to the Company was not made on a timely basis, deferred income of $1.9 million
was recorded, pending receipt of future payments on the note. If all payments
are received, a gain of $1.3 million, net of tax, will be recorded on the
discontinued operations line. A reserve was also established for settlement of
operational expenses subsequent to finalization of the contract of sale.

7.   New Product Acquisition

During the second quarter 1998, the Company acquired the US rights to market
PENTASA, a patented gastrointestinal drug for ulcerative colitis, from Hoechst
Marion Roussel (HMR) for a net cost of $135. million. The transaction involved a
payment to HMR of $141.0 million and the Company received a note receivable for
$6.2 million from HMR. A term loan for $125.0 million was taken out to help
finance the transaction.

                                     - 8 -
<PAGE>
 
FORWARD LOOKING STATEMENTS

Certain statements included in Footnotes #1 and #5 and Items 2 and 6 of this
form 10-Q are intended to be, and are hereby identified as, forward looking
statements for purposes of the safe harbor provided by Section 21E of the
Securities Exchange Act of 1934, as amended, and Section 27A of the Securities
Act of 1933, as amended.

The Registrant cautions readers that forward looking statements, including,
without limitation, those relating to the Registrant's future business
prospects, revenues, cost of sales, intangible dispositions and write-offs,
continuing operations and discontinued operations, and liquidity and capital
resources, are subject to certain risks and uncertainties, including, without
limitation, the ability of the Registrant to secure regulatory approval in the
United States and in foreign jurisdictions for the Registrant's developmental
pipeline drugs, the efforts of the Registrant's competitors and the introduction
of rival pharmaceutical products which may prove to be more effective than the
Registrant's products, general market conditions, the availability of capital,
and the uncertainty over the future direction of the healthcare industry, that
could cause actual results to differ materially from those indicated in the
forward looking statements.




















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Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this amendment to be signed on its behalf by the
undersigned, thereunto duly authorized.


Date:   November 25, 1998          /s/    Peter M. Rogalin      
        -----------------          Peter M. Rogalin
                                   Vice President and Treasurer



Date:   November 25, 1998          /s/    Peter M. Rogalin      
        -----------------          Peter M. Rogalin
                                   Chief Accounting Officer

















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