**Exhibit 31.1****CERTIFICATIONS**I, Adrian Adams, Chief Executive Officer of Kos Pharmaceuticals, Inc., certify that:1. I have reviewed this annual report on Form 10-K of Kos Pharmaceuticals, Inc; 2. Based on my knowledge, this annual report does not contain any untrue statement of a materialfact or omit to state a material fact necessary to make the statements made, in light of thecircumstances under which the statements were made, not misleading with respect to the periodcovered by this annual report; 3. Based on my knowledge, the financial statements, and other financial information included inthis annual report, fairly present in all material respects the financial condition, resultsof operations and cash flows of the registrant as of, and for, the periods presented in thisannual report; 4. The registrant’s other certifying officers and I are responsible for establishing andmaintaining disclosure controls and procedures (as defined in Exchange Act Rules 13a-15(e) and15d-15(e)) and internal control over financial reporting (as defined in Exchange Act Rules13a-15(f) and 15-15(f)) for the registrant and we have: (a) Designed such disclosure controls and procedures or caused such disclosure controls andprocedures to be designed under our supervision, to ensure that material informationrelating to the registrant, including its consolidated subsidiaries, is made known to us byothers within those entities, particularly during the period in which this annual report isbeing prepared;  (b) Designed such internal control over financial reporting, or caused such internalcontrol over financial reporting to be designed under our supervision, to providereasonable assurance regarding the reliability of financial reporting and the preparationof financial statements for external purposes in accordance with generally accountingprinciples;  (c) Evaluated the effectiveness of the registrant’s disclosure controls and procedures andpresented in this report our conclusions about the effectiveness of the disclosure controlsand procedures, as of the end of the period covered by this report based on suchevaluation; and  (d) Disclosed in this report any change in the registrant’s internal control over financialreporting that occurred during the registrant’s most recent fiscal quarter (theregistrant’s fourth fiscal quarter in the case of the annual report) that has materiallyaffected, or is reasonably likely to materially affect, the registrant’s internal controlover financial reporting.5. The registrant’s other certifying officers and I have disclosed, based on our most recentevaluation of internal control over financial reporting, to the registrant’s auditors and theaudit committee of the registrant’s board of directors (or persons performing the equivalentfunction): (a) All significant deficiencies and material weaknesses in the design or operation ofinternal control which are reasonably likely to adversely affect the registrant’s abilityto record, process, summarize and report financial information; and  (b) Any fraud, whether or not material, that involves management or other employees whohave a significant role in the registrant’s internal controls over financial reporting.Date: March 16, 2006                                                                     /s/ Adrian Adams   Adrian Adams   Chief Executive Officer