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Preliminary Proxy Statement

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COLLEGIUM PHARMACEUTICAL, INC.

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MMMMMMMMMMMM + MMMMMMM C 1234567890 The Sample Company 000004 ENDORSEMENT_LINE______________ SACKPACK_____________ MR A SAMPLE DESIGNATION (IF ANY) ADD 1 ADD 2 ADD 3 ADD 4 ADD 5 ADD 6 Online Go to www.envisionreports.com/COPI or scan the QR code — login details are located in the shaded bar below. Important Notice Regarding the Availability of Proxy Materials for the Collegium Pharmaceutical, Inc. Meeting to be Held on May 20, 2020 Under Securities and Exchange Commission rules, you are receiving this notice that the proxy materials for the annual shareholders’ meeting are available on the Internet. Follow the instructions below to view the materials and vote online or request a copy. The items to be voted on and location of the annual meeting are on the reverse side. Your vote is important! This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. We encourage you to access and review all of the important information contained in the proxy materials before voting. The proxy statement and annual report to shareholders are available at: www.envisionreports.com/COPI Easy Online Access — View your proxy materials and vote. Step 1: Step 2: Step 3: Step 4: Step 5: Go to www.envisionreports.com/COPI. Click on Cast Your Vote or Request Materials. Follow the instructions on the screen to log in. Make your selections as instructed on each screen for your delivery preferences. Vote your shares. When you go online, you can also help the environment by consenting to receive electronic delivery of future materials. Obtaining a Copy of the Proxy Materials – If you want to receive a copy of these documents, you must request one. There is no charge to you for requesting a copy. Please make your request for a copy as instructed on the reverse side on or before May 10, 2020 to facilitate timely delivery. + 2 N O T C O Y 038BFC MMMMMMMMM Shareholder Meeting Notice 1234 5678 9012 345

Collegium Pharmaceutical, Inc.’s Annual Meeting of Shareholders will be held on May 20, 2020 virtually via live webcast www.meetingcenter.io/200054000 at 8:30 a.m. Eastern Time. The password for the meeting is COPI2020. Proposals to be voted on at the meeting are listed below along with the Board of Directors’ recommendations. The Board of Directors recommend a vote FOR all the nominees listed and FOR Proposals 2, 3, 4 and 5: 1. 2. 3. Election of Joseph Ciaffoni, Michael Heffernan, R.Ph., and Gino Santini as Class II directors to hold office until the 2023 Annual Meeting. Approval of, on an advisory basis, the compensation of the Company’s named executive officers Approval of a proposed amendment and restatement of the Company’s Second Amended and Restated Articles of Incorporation to declassify the Company’s Board of Directors Approval of a proposed amendment and restatement of the Company’s Second Amended and Restated Articles of Incorporation to eliminate the supermajority voting requirement for amendments to the Company’s Amended and Restated Bylaws Ratification of Deloitte & Touche LLP as the Company’s independent registered public accounting firm for the fiscal year ending December 31, 2020 4. 5. PLEASE NOTE – YOU CANNOT VOTE BY RETURNING THIS NOTICE. To vote your shares you must vote online or request a paper copy of the proxy materials to receive a proxy card. If you wish to attend and vote at the meeting, please bring this notice with you. Here’s how to order a copy of the proxy materials and select delivery preferences: Current and future delivery requests can be submitted using the options below. If you request an email copy, you will receive an email with a link to the current meeting materials. PLEASE NOTE: You must use the number in the shaded bar on the reverse side when requesting a copy of the proxy materials. — — — Internet – Go to www.envisionreports.com/COPI. Click Cast Your Vote or Request Materials. Phone – Call us free of charge at 1-866-641-4276. Email – Send an email to investorvote@computershare.com with “Proxy Materials Collegium Pharmaceutical, Inc.” in the subject line. Include your full name and address, plus the number located in the shaded bar on the reverse side, and state that you want a paper copy of the meeting materials. To facilitate timely delivery, all requests for a paper copy of proxy materials must be received by May 10, 2020. Shareholder Meeting Notice