VOTING AGREEMENT

Exhibit 2.3

VOTING AGREEMENT

THIS VOTING AGREEMENT (this “Agreement”) is entered into as of August 29, 2008, by and between TRANSCEPT PHARMACEUTICALS, INC., a Delaware corporation (“Merger Partner”), and                          (collectively, the “Stockholder”).

RECITALS

A. Stockholder is a holder of record and the “beneficial owner” (within the meaning of Rule 13d-3 under the Securities Exchange Act of 1934) of certain shares of common stock of Novacea, Inc., a Delaware corporation (the “Company”).

B. The Company, Pivot Acquisition, Inc., a Delaware corporation (“Merger Sub”), and Merger Partner are entering into an Agreement and Plan of Merger and Reorganization of even date herewith (the “Merger Agreement”) which provides (subject to the conditions set forth therein) for the merger of Merger Sub into Merger Partner (the “Merger”).

C. In the Merger, each outstanding share of common stock of Merger Partner is to be converted into the right to receive that number of shares of common stock of the Company as set forth in Section 1.5(a)(iii) of the Merger Agreement.

D. Stockholder is entering into this Agreement in order to induce Merger Partner to enter into the Merger Agreement.

AGREEMENT

The parties to this Agreement, intending to be legally bound, agree as follows:

SECTION 1. CERTAIN DEFINITIONS

For purposes of this Agreement:

(a) “Company Common Stock” shall mean the common stock, par value $0.001 per share, of the Company.

(b) Stockholder shall be deemed to “Own” or to have acquired “Ownership” of a security if Stockholder: (i) is the record owner of such security; or (ii) is the “beneficial owner” (within the meaning of Rule 13d-3 under the Securities Exchange Act of 1934) of such security.

(c) “Person” shall mean any (i) individual, (ii) corporation, limited liability company, partnership or other entity, or (iii) governmental authority.

(d) “Proxy Expiration Date” shall mean the earlier of (i) the date upon which the Merger Agreement is terminated, or (ii) the date upon which the Merger becomes effective.

(e) “Subject Securities” shall mean: (i) all securities of the Company (including all shares of Company Common Stock and all options, warrants and other rights to acquire shares of Company Common Stock) Owned by Stockholder as of the date of this Agreement; and (ii) all additional securities of the Company (including all additional shares of Company Common Stock and all additional options, warrants and other rights to acquire shares of Company Common Stock) of which Stockholder acquires Ownership during the period from the date of this Agreement through the Proxy Expiration Date.

(f) A Person shall be deemed to have a effected a “Transfer” of a security if such Person directly or indirectly: (i) sells, pledges, encumbers, grants an option with respect to, transfers or disposes of such security or any interest in such security to any Person; (ii) enters into an agreement or commitment contemplating the possible sale of, pledge of, encumbrance of, grant of an option with respect to, transfer of or disposition of such security or any interest therein to any Person; or (iii) reduces such Person’s beneficial ownership of, interest in or risk relating to such security.

(g) Capitalized terms used but not otherwise defined in this Agreement have the meanings assigned to such terms in the Merger Agreement.

SECTION 2. TRANSFER OF SUBJECT SECURITIES AND VOTING RIGHTS

2.1 Restriction on Transfer of Subject Securities. Subject to Section 2.3, during the period from the date of this Agreement through the Proxy Expiration Date, Stockholder shall not, directly or indirectly, cause or permit any Transfer of any of the Subject Securities to be effected.

2.2 Restriction on Transfer of Voting Rights. During the period from the date of this Agreement through the Proxy Expiration Date, Stockholder shall ensure that: (a) none of the Subject Securities is deposited into a voting trust; and (b) no proxy is granted, and no other voting agreement or similar agreement is entered into, with respect to any of the Subject Securities.

2.3 Permitted Transfers. Section 2.1 shall not prohibit a transfer of Subject Securities by Stockholder (a) if Stockholder is an individual (i) to any member of Stockholder’s immediate family, or to a trust for the benefit of Stockholder or any member of Stockholder’s immediate family, or (ii) upon the death of Stockholder, or (b) if Stockholder is a partnership or limited liability company, to one or more partners or members of Stockholder or to an affiliated corporation under common control with Stockholder; provided, however, that a transfer referred to in this sentence shall be permitted only if, as a precondition to such transfer, the transferee agrees in a writing, reasonably satisfactory in form and substance to Merger Partner, to be bound by all of the terms of this Agreement.

2

SECTION 3. VOTING OF SHARES

3.1 Voting Covenant. Stockholder hereby agrees that, prior to the Proxy Expiration Date, at any meeting of the stockholders of the Company, however called, and in any written action by consent of stockholders of the Company, unless otherwise provided in the Merger Agreement or directed in writing by Merger Partner, Stockholder shall cause the Subject Securities to be voted (to the extent such Subject Securities may be so voted):

(a) in favor of the Merger, the execution and delivery by the Company of the Merger Agreement and the adoption and approval of the Merger Agreement and the terms thereof, in favor of each of the other actions contemplated by the Merger Agreement and in favor of any action in furtherance of any of the foregoing;

(b) against any action or agreement that would result in a breach of any representation, warranty, covenant or obligation of the Company in the Merger Agreement; and

(c) against the following actions (other than the Merger and the transactions contemplated by the Merger Agreement): (A) any extraordinary corporate transaction, such as a merger, consolidation or other business combination involving the Company or any subsidiary of the Company; (B) any sale, lease, sublease, license, sublicense or transfer of a material portion of the rights or other assets of the Company or any subsidiary of the Company; (C) any change in a majority of the board of directors of the Company; (D) any amendment to the Company’s certificate of incorporation or bylaws; (E) any material change in the capitalization of the Company or the Company’s corporate structure; and (F) any other action which is intended, or could reasonably be expected, to impede, interfere with, delay, postpone, discourage or adversely affect the Merger or any of the other transactions contemplated by the Merger Agreement or this Agreement.

Prior to the Proxy Expiration Date, Stockholder shall not enter into any agreement or understanding with any Person to vote or give instructions in any manner inconsistent with clause “(a)”, clause “(b)” or clause “(c)” of the preceding sentence.

3.2 PROXY; FURTHER ASSURANCES

(a) Contemporaneously with the execution of this Agreement: (i) Stockholder shall deliver to Merger Partner a proxy in the form attached to this Agreement as Exhibit A, which shall be irrevocable to the fullest extent permitted by law (at all times prior to the Proxy Expiration Date) with respect to the shares referred to therein (the “Proxy”); and (ii) Stockholder shall cause to be delivered to Merger Partner an additional proxy (in the form attached hereto as Exhibit A) executed on behalf of the record owner of any outstanding shares of Company Common Stock that are owned beneficially (within the meaning of Rule 13d-3 under the Securities Exchange Act of 1934), but not of record, by Stockholder.

(b) Stockholder shall perform such further acts and execute such further proxies and other documents and instruments as may reasonably be required to vest in Merger Partner the power to carry out and give effect to the provisions of this Agreement.

3

(c) Stockholder shall not enter into any tender, voting or other such agreement, or grant a proxy or power of attorney, with respect to the Subject Securities that is inconsistent with this Agreement or otherwise take any other action with respect to the Subject Securities that would in any way restrict, limit or interfere with the performance of Stockholder’s obligations hereunder or the transactions contemplated hereby.

SECTION 4. NO SOLICITATION

Stockholder hereby represents and warrants that he, she or it has read Section 4.5 of the Merger Agreement and agrees to be bound by the provisions of such section to the extent applicable to Stockholder.

SECTION 5. NO OWNERSHIP INTEREST

Nothing contained in this Agreement shall be deemed to vest in Merger Partner any direct or indirect ownership or incidence of ownership of or with respect to any Subject Securities. All rights, ownership and economic benefits of and relating to the Subject Securities shall remain vested in and belong to Stockholder, and Merger Partner shall have no authority to exercise any power or authority to direct Stockholder in the voting of any of the Subject Securities, except as otherwise specifically provided herein, or in the performance of Stockholder’s duties or responsibilities as a stockholder of the Company.

SECTION 6. REPRESENTATIONS AND WARRANTIES OF STOCKHOLDER

Stockholder hereby represents and warrants to Merger Partner as of the date hereof as follows:

6.1 Authorization, etc. Stockholder has the requisite right, power, authority and capacity to execute and deliver this Agreement and the Proxy and to perform Stockholder’s obligations hereunder and thereunder. This Agreement and the Proxy have been duly executed and delivered by Stockholder and constitute legal, valid and binding obligations of Stockholder, enforceable against Stockholder in accordance with their terms, subject to (i) laws of general application relating to bankruptcy, insolvency and the relief of debtors, and (ii) rules of law governing specific performance, injunctive relief and other equitable remedies. If Stockholder is a corporation, then Stockholder is a corporation duly organized and validly existing under the laws of the jurisdiction in which it was organized. If Stockholder is a general or limited partnership, then Stockholder is a partnership duly organized and validly existing under the laws of the jurisdiction in which it was organized. If Stockholder is a limited liability company, then Stockholder is a limited liability company duly organized and validly existing under the laws of the jurisdiction in which it was organized.

6.2 No Conflicts or Consents.

(a) To the Stockholder’s knowledge (but without having conducted any independent investigation), the execution and delivery of this Agreement and the Proxy by Stockholder do not, and the performance of this Agreement and the Proxy by

4

Stockholder will not: (i) conflict with or violate any material law, rule, regulation, order, decree or judgment applicable to Stockholder or by which Stockholder or any of Stockholder’s properties is or may be bound or affected; or (ii) result in or constitute (with or without notice or lapse of time) any breach of or default under, or give to any other Person (with or without notice or lapse of time) any right of termination, amendment, acceleration or cancellation of, or result (with or without notice or lapse of time) in the creation of any encumbrance or restriction on any of the Subject Securities pursuant to, any material Contract to which Stockholder is a party or by which Stockholder or any of Stockholder’s affiliates or properties is or may be bound or affected.

(b) The execution and delivery of this Agreement and the Proxy by Stockholder do not, and the performance of this Agreement and the Proxy by Stockholder will not, require any consent or approval of any Person. The execution and delivery of any additional proxy pursuant to Section 3.2(a)(ii) with respect to any shares of Company Common Stock that are owned beneficially but not of record by Stockholder do not, and the performance of any such additional proxy will not, require any consent or approval of any Person.

6.3 Title to Securities. As of the date of this Agreement, and except as provided in this Agreement: (a) Stockholder holds of record (free and clear of any encumbrances or restrictions) the number of outstanding shares of Company Common Stock set forth under the heading “Shares Held of Record” on the signature page hereof; (b) Stockholder holds (free and clear of any encumbrances or restrictions other than restrictions under applicable securities laws) the options, warrants and other rights to acquire shares of Company Common Stock set forth under the heading “Options and Other Rights” on the signature page hereof; and (c) Stockholder Owns the additional securities of the Company set forth under the heading “Additional Securities Beneficially Owned” on the signature page hereof.

6.4 Accuracy of Representations. The representations and warranties contained in this Agreement are accurate in all respects as of the date of this Agreement, and, to the knowledge of Stockholders, will be accurate in all respects at all times through and including the Proxy Expiration Date as if made as of any such time or date.

SECTION 7. ADDITIONAL COVENANTS OF STOCKHOLDER

7.1 Stockholder Information. Stockholder hereby agrees to permit Merger Partner and the Company, an intended third party beneficiary to this Section 7.1, to publish and disclose in the Form S-4 Stockholder’s identity and ownership of shares of Company Common Stock and the nature of Stockholder’s commitments, arrangements and understandings under this Agreement.

7.2 Further Assurances. From time to time and without additional consideration, Stockholder shall execute and deliver, or cause to be executed and delivered, such additional transfers, assignments, endorsements, proxies, consents and other instruments, and shall (at Stockholder’s sole expense) take such further actions, as Merger Partner may reasonably request for the purpose of carrying out and furthering the intent of this Agreement.

5

SECTION 8. MISCELLANEOUS

8.1 Termination. This Agreement, and all rights and obligations of the parties hereunder, shall terminate upon the earlier to occur of (i) the effective time of the Merger or (ii) the termination of the Merger Agreement.

8.2 Survival of Representations, Warranties and Agreements. All representations, warranties, covenants and agreements made by Stockholder in this Agreement shall survive the Proxy Expiration Date until the earlier to occur of (a) the effective time of the Merger or (a) a period of one year from the date hereof.

8.3 Expenses. All costs and expenses incurred in connection with the transactions contemplated by this Agreement shall be paid by the party incurring such costs and expenses.

8.4 Notices. Any notice or other communication required or permitted to be delivered to either party under this Agreement shall be in writing and shall be deemed properly delivered, given and received when received at the address or facsimile telephone number set forth beneath the name of such party below (or at such other address or facsimile telephone number as such party shall have specified in a written notice given to the other party):

if to Stockholder:

at the address set forth on the signature page hereof; and

if to Merger Partner:

Transcept Pharmaceutical Inc.

1003 W. Cutting Blvd.

Suite #110

Point Richmond, California 94804

Attn: Chief Executive Officer

Fax: 510 ###-###-####

8.5 Severability. Any term or provision of this Agreement that is invalid or unenforceable in any situation in any jurisdiction shall not affect the validity or enforceability of the remaining terms and provisions hereof or the validity or enforceability of the offending term or provision in any other situation or in any other jurisdiction. If the final judgment of a court of competent jurisdiction declares that any term or provision hereof is invalid or unenforceable, the parties hereto agree that the court making such determination shall have the power to limit the term or provision, to delete specific words or phrases, or to replace any invalid or unenforceable term or provision with a term or provision that is valid and enforceable and that comes closest to expressing the intention of the invalid or unenforceable term or provision, and this Agreement shall be enforceable as so modified. In the event such court does not exercise the power granted to it in the prior sentence, the parties hereto agree to replace such invalid or unenforceable term or provision with a valid and enforceable term or provision that will achieve, to the extent possible, the economic, business and other purposes of such invalid or unenforceable term.

6

8.6 Entire Agreement. This Agreement, the Proxy and any other documents delivered by the parties in connection herewith constitute the entire agreement between the parties with respect to the subject matter hereof and thereof and supersede all prior agreements and understandings between the parties with respect thereto. No addition to or modification of any provision of this Agreement shall be binding upon either party unless made in writing and signed by both parties.

8.7 Assignment; Binding Effect. Except as provided herein, neither this Agreement nor any of the interests or obligations hereunder may be assigned or delegated by Stockholder, and any attempted or purported assignment or delegation of any of such interests or obligations shall be void. Subject to the preceding sentence, this Agreement shall be binding upon Stockholder and Stockholder’s heirs, estate, executors and personal representatives and Stockholder’s successors and assigns, and shall inure to the benefit of Merger Partner and its successors and assigns. Without limiting any of the restrictions set forth in Section 2 or elsewhere in this Agreement, this Agreement shall be binding upon any Person to whom any Subject Securities are transferred. Nothing in this Agreement is intended to confer on any Person (other than Merger Partner and its successors and assigns) any rights or remedies of any nature.

8.8 Independence of Obligations. The covenants and obligations of Stockholder set forth in this Agreement shall be construed as independent of any other agreement or arrangement between Stockholder, on the one hand, and the Company or Merger Partner, on the other. The existence of any claim or cause of action by Stockholder against the Company or Merger Partner shall not constitute a defense to the enforcement of any of such covenants or obligations against Stockholder.

8.9 Specific Performance. The parties agree that irreparable damage would occur in the event that any of the provisions of this Agreement or the Proxy were not performed in accordance with its specific terms or were otherwise breached. Stockholder agrees that, in the event of any breach or threatened breach by Stockholder of any covenant or obligation contained in this Agreement or in the Proxy, Merger Partner shall be entitled (in addition to any other remedy that may be available to it, including monetary damages) to seek and obtain (a) a decree or order of specific performance to enforce the observance and performance of such covenant or obligation, and (b) an injunction restraining such breach or threatened breach. Stockholder further agrees that neither Merger Partner nor any other Person shall be required to obtain, furnish or post any bond or similar instrument in connection with or as a condition to obtaining any remedy referred to in this Section 8.9, and Stockholder irrevocably waives any right he or it may have to require the obtaining, furnishing or posting of any such bond or similar instrument.

8.10 Non-Exclusivity. The rights and remedies of Merger Partner under this Agreement are not exclusive of or limited by any other rights or remedies which it may have, whether at law, in equity, by contract or otherwise, all of which shall be cumulative (and not alternative). Without limiting the generality of the foregoing, the rights and remedies of Merger Partner under this Agreement, and the obligations and liabilities of Stockholder under this Agreement, are in addition to their respective rights, remedies, obligations and liabilities under common law requirements and under all applicable statutes, rules and regulations. Nothing in this Agreement shall

7

limit any of Stockholder’s obligations, or the rights or remedies of Merger Partner, under any agreement between Merger Partner and Stockholder; and nothing in any such agreement shall limit any of Stockholder’s obligations, or any of the rights or remedies of Merger Partner, under this Agreement.

8.11 Governing Law; Jurisdiction; Waiver of Jury Trial.

(a) This Agreement shall be governed by, and construed in accordance with, the laws of the State of Delaware, regardless of the laws that might otherwise govern under applicable principles of conflicts of laws thereof. In any action between the parties arising out of or relating to this Agreement or any of the transactions contemplated by this Agreement each of the parties irrevocably and unconditionally consents and submits to the jurisdiction and venue of the state and federal courts located in the Northern District of California and the State of Delaware.

(b) STOCKHOLDER IRREVOCABLY WAIVES THE RIGHT TO A JURY TRIAL IN CONNECTION WITH ANY LEGAL PROCEEDING RELATING TO THIS AGREEMENT OR THE PROXY OR THE ENFORCEMENT OF ANY PROVISION OF THIS AGREEMENT OR THE PROXY.

8.12 Counterparts. This Agreement may be executed in separate counterparts, each of which when so executed and delivered shall be an original, but all such counterparts shall together constitute one and the same instrument.

8.13 Captions. The captions contained in this Agreement are for convenience of reference only, shall not be deemed to be a part of this Agreement and shall not be referred to in connection with the construction or interpretation of this Agreement.

8.14 Attorneys’ Fees. If any legal action or other legal proceeding relating to this Agreement or the enforcement of any provision of this Agreement is brought by either party against the other, the prevailing party shall be entitled to recover reasonable attorneys’ fees, costs and disbursements (in addition to any other relief to which the prevailing party may be entitled).

8.15 Waiver. No failure on the part of Merger Partner to exercise any power, right, privilege or remedy under this Agreement, and no delay on the part of Merger Partner in exercising any power, right, privilege or remedy under this Agreement, shall operate as a waiver of such power, right, privilege or remedy; and no single or partial exercise of any such power, right, privilege or remedy shall preclude any other or further exercise thereof or of any other power, right, privilege or remedy. Merger Partner shall not be deemed to have waived any claim available to Merger Partner arising out of this Agreement, or any power, right, privilege or remedy of Merger Partner under this Agreement, unless the waiver of such claim, power, right, privilege or remedy is expressly set forth in a written instrument duly executed and delivered on behalf of Merger Partner; and any such waiver shall not be applicable or have any effect except in the specific instance in which it is given.

8

8.16 Construction.

(a) For purposes of this Agreement, whenever the context requires: the singular number shall include the plural, and vice versa; the masculine gender shall include the feminine and neuter genders; the feminine gender shall include the masculine and neuter genders; and the neuter gender shall include masculine and feminine genders.

(b) The parties agree that any rule of construction to the effect that ambiguities are to be resolved against the drafting party shall not be applied in the construction or interpretation of this Agreement.

(c) As used in this Agreement, the words “include” and “including,” and variations thereof, shall not be deemed to be terms of limitation, but rather shall be deemed to be followed by the words “without limitation.”

(d) Except as otherwise indicated, all references in this Agreement to “Sections” and “Exhibits” are intended to refer to Sections of this Agreement and Exhibits to this Agreement.

[Remainder of page intentionally left blank.]

9

IN WITNESS WHEREOF, Merger Partner and Stockholder have caused this Agreement to be executed as of the date first written above.

Signature Page to Stockholder Agreement

EXHIBIT A

FORM OF IRREVOCABLE PROXY

The undersigned stockholder (the “Stockholder”) of NOVACEA, INC., a Delaware corporation (the “Company”), hereby irrevocably (to the fullest extent permitted by law) appoints and constitutes                     ,                      and TRANSCEPT PHARMACEUTICALS, INC., a Delaware corporation (“Merger Partner”), and each of them, the attorneys and proxies of the Stockholder, with full power of substitution and resubstitution, to the full extent of the Stockholder’s rights with respect to (i) the outstanding shares of capital stock of the Company owned of record by the Stockholder as of the date of this proxy, which shares are specified on the final page of this proxy, and (ii) any and all other shares of capital stock of the Company which the Stockholder may acquire on or after the date hereof. (The shares of the capital stock of the Company referred to in clauses “(i)” and “(ii)” of the immediately preceding sentence are collectively referred to as the “Shares.”) Upon the execution hereof, all prior proxies given by the Stockholder with respect to any of the Shares are hereby revoked, and the Stockholder agrees that no subsequent proxies will be given with respect to any of the Shares.

This proxy is irrevocable, is coupled with an interest and is granted in connection with, and as security for, the Voting Agreement, dated as of the date hereof, between Merger Partner and the Stockholder (the “Voting Agreement”), and is granted in consideration of Merger Partner entering into the Agreement and Plan of Merger and Reorganization, dated as of the date hereof, among the Company, Pivot Acquisition, Inc., a wholly-owned subsidiary of the Company, and Merger Partner (the “Merger Agreement”). This proxy will terminate on the Proxy Expiration Date (as defined in the Voting Agreement).

The attorneys and proxies named above will be empowered, and may exercise this proxy, to vote the Shares at any time until the earlier to occur of the termination of the Merger Agreement or the effective time of the merger contemplated thereby (the “Merger”) at any meeting of the stockholders of the Company, however called, and in connection with any written action by consent of stockholders of the Company:

(i) in favor of the Merger, the execution and delivery by the Company of the Merger Agreement and the adoption and approval of the Merger Agreement and the terms thereof, in favor of each of the other actions contemplated by the Merger Agreement and in favor of any action in furtherance of any of the foregoing; and

(ii) against any action or agreement that would result in a breach of any representation, warranty, covenant or obligation of the Company in the Merger Agreement; and

(iii) against the following actions (other than the Merger and the other transactions contemplated by the Merger Agreement): (A) any extraordinary corporate transaction, such as a merger, consolidation or other business combination involving the Company or any subsidiary of the Company; (B) any sale, lease, sublease, license, sublicense or transfer of a material portion of the rights or other assets of the Company or any subsidiary of the Company; (C) any reorganization, recapitalization, dissolution or liquidation of the Company or any subsidiary of the Company; (D) any change in a majority of the board of directors of the Company; (E) any amendment to the Company’s certificate of incorporation or bylaws; (F) any material

A-1

change in the capitalization of the Company or the Company’s corporate structure; and (G) any other action which is intended, or could reasonably be expected to impede, interfere with, delay, postpone, discourage or adversely affect the Merger or any of the other transactions contemplated by the Merger Agreement or the Voting Agreement.

The Stockholder may vote the Shares on all other matters not referred to in this proxy, and the attorneys and proxies named above may not exercise this proxy with respect to such other matters.

This proxy shall be binding upon the heirs, estate, executors, personal representatives, successors and assigns of the Stockholder (including any transferee of any of the Shares).

Any term or provision of this proxy that is invalid or unenforceable in any situation in any jurisdiction shall not affect the validity or enforceability of the remaining terms and provisions hereof or the validity or enforceability of the offending term or provision in any other situation or in any other jurisdiction. If the final judgment of a court of competent jurisdiction declares that any term or provision hereof is invalid or unenforceable, the Stockholder agrees that the court making such determination shall have the power to limit the term or provision, to delete specific words or phrases, or to replace any invalid or unenforceable term or provision with a term or provision that is valid and enforceable and that comes closest to expressing the intention of the invalid or unenforceable term or provision, and this proxy shall be enforceable as so modified. In the event such court does not exercise the power granted to it in the prior sentence, the parties hereto agree to replace such invalid or unenforceable term or provision with a valid and enforceable term or provision that will achieve, to the extent possible, the economic, business and other purposes of such invalid or unenforceable term.

Dated: August 29, 2008