NOT FOR RELEASE, PUBLICATION OR DISTRIBUTION, IN WHOLE OR IN PART, IN, INTO OR FROM ANY JURISDICTION WHERE TO DO SO WOULD CONSTITUTE A VIOLATION OF THE RELEVANT LAWS OR REGULATIONS OF SUCH JURISDICTION

In accordance with Rule 2.12 of the Irish Takeover Panel Act, 1997, Takeover Rules, 2022 (the “Irish Takeover Rules”), Horizon confirms that, as of the close of business on February 23, 2023, Horizon’s issued ordinary share capital is comprised of 228,832,390 ordinary shares, nominal value $0.0001 per share (the “Ordinary Shares”) and 40,000 deferred ordinary shares of €1.00 each. The Company has 384,366 Ordinary Shares which are held as treasury shares. The Ordinary Shares are admitted to trading on the Nasdaq Global Select Market under the ticker symbol “HZNP”. The International Securities Identification Number for these securities is IE00BQPVQZ61.

The Company confirms that as of February 23, 2023, there were outstanding options to purchase up to 4,758,722 Ordinary Shares and outstanding restricted stock units and performance stock units conferring on their holders vested or unvested rights to convert into, or receive, up to an aggregate of 1,810,200 Ordinary Shares and incremental restricted stock units to be issued of 4,361,896 Ordinary Shares.

About Horizon

Horizon Therapeutics plc is focused on the discovery, development and commercialization of medicines that address critical needs for people impacted by rare, autoimmune and severe inflammatory diseases. Our pipeline is purposeful: We apply scientific expertise and courage to bring clinically meaningful therapies to patients. We believe science and compassion must work together to transform lives. For more information on how we go to incredible lengths to impact lives, please visit www.horizontherapeutics.com.

Enquiries

Horizon Investor Relations

investor-relations@horizontherapeutics.com

Statement Required by the Irish Takeover Rules

The directors of Horizon accept responsibility for the information contained in this document. To the best of the knowledge and belief of the directors of Horizon (who have taken all reasonable care to ensure such is the case), the information contained in this document for which they respectively accept responsibility is in accordance with the facts and does not omit anything likely to affect the import of such information.

Dealing Disclosure Requirements

Under the provisions of Rule 8.3(b) of the Irish Takeover Rules, if any person is, or becomes, ‘interested’ (directly or indirectly) in 1% or more of any class of ‘relevant securities’ of Horizon (including by means of an option in respect of, or a derivative referenced to, any such ‘relevant securities’), that person must publicly disclose all ‘dealings’ in any ‘relevant securities’ of Horizon during the ‘offer period,’ by not later than 3:30 p.m. (E.T.) on the ‘business day’ following the date of the relevant transaction.

If two or more persons co-operate on the basis of any agreement either express or tacit, either oral or written, to acquire an ‘interest’ in ‘relevant securities’ of Horizon, they will be deemed to be a single person for the purpose of Rule 8.3 of the Irish Takeover Rules.

A disclosure table, giving details of the companies in whose ‘relevant securities’ and ‘dealings’ should be disclosed can be found on the Irish Takeover Panel’s website at www.irishtakeoverpanel.ie.

‘Interests’ in securities arise, in summary, when a person has long economic exposure, whether conditional or absolute, to changes in the price of securities. In particular, a person will be treated as having an ‘interest’ by virtue of the ownership or control of securities, or by virtue of any option in respect of, or derivative referenced to, securities.

Terms in quotation marks are defined in the Irish Takeover Rules, which can be found on the Irish Takeover Panel’s website. If you are in any doubt as to whether or not you are required to disclose a ‘dealing’ under Rule 8, please consult the Irish Takeover Panel’s website at www.irishtakeoverpanel.ie or contact the Irish Takeover Panel at telephone number +353 1 678 9020.

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