Horizon Therapeutics
HZNP

From March 31, 2023 to June 30, 2023 Issuance of ordinary shares in conjunction with the Employee Share Purchase Plan Issuance of ordinary shares in conjunction with the exercise of stock options and the vesting of restricted stock units and performance stock units

From December 31, 2022 to March 31, 2023 Issuance of ordinary shares in conjunction with the exercise of stock options and the vesting of restricted stock units and performance stock units

From December 31, 2021 to December 31, 2022 Issuance of ordinary shares in conjunction with Employee Share Purchase Plan Issuance of ordinary shares in conjunction with the exercise of stock options and the vesting of restricted stock units and performance stock units Repurchase of ordinary shares

From June 30, 2022 to September 30, 2022 Issuance of ordinary shares in conjunction with the exercise of stock options and the vesting of restricted stock units and performance stock units Repurchase of ordinary shares

From March 31, 2022 to June 30, 2022 Issuance of ordinary shares in conjunction with Employee Share Purchase Plan Issuance of ordinary shares in conjunction with the exercise of stock options and the vesting of restricted stock units and performance stock units

From December 31, 2021 to March 31, 2022 Issuance of ordinary shares in conjunction with the exercise of stock options and the vesting of restricted stock units and performance stock units

From December 31, 2020 to December 31, 2021 Issuance of ordinary shares in conjunction with the exercise of stock options and the vesting of restricted stock and performance stock units Issuance of ordinary shares in conjunction with Employee Share Purchase Plan

From June 30, 2021 to September 30, 2021 Issuance of ordinary shares in conjunction with the exercise of stock options and the vesting of restricted stock and performance stock units

From March 31, 2021 to June 30, 2021 Issuance of ordinary shares in conjunction with the exercise of stock options and the vesting of restricted stock and performance stock units Issuance of ordinary shares in conjunction with Employee Share Purchase Plan

from December 31, 2020 to March 31, 2021 Issuance of ordinary shares in conjunction with the exercise of stock options and the vesting of restricted stock and performance stock units

From December 31,2019 to December 31,2020 Issuance of ordinary shares in conjunction with ESPP program Issuance of ordinary shares - public offering Issuance of ordinary shares in conjunction with the exercise of stock options and the vesting of restricted stock and performance stock units Issuance of ordinary shares in conjunction with Exchangeable Senior Notes

From June 30, 2020 to September 30, 2020 Issuance of ordinary shares - public offering Issuance of ordinary shares in conjunction with the exercise of stock options and the vesting of restricted stock and performance stock units Issuance of ordinary shares in conjunction with Exchangeable Senior Notes

1.Ordinary shares offered by the company 11,800,000 ordinary shares. 2.Unless indicate otherwise, all information in this prospectus supplement is based on 199,607,441 ordinary shares outstanding as of June 30, 2020.

From March 31, 2020 to June 30, 2020 Issuance of ordinary shares in conjunction with the exercise of stock options and the vesting of restricted stock and performance stock units Issuance of ordinary shares in conjunction with ESPP program Issuance of ordinary shares in conjunction with Exchangeable Senior Notes

From December 31, 2019 to March 31, 2020 Issuance of ordinary shares in conjunction with vesting of restricted stock units, performance stock units and stock option exercises

From December 31, 2018 to December 31, 2019 Issuance of ordinary shares - public offering Issuance of ordinary shares in conjunction with vesting of restricted stock units, performance stock units and stock option exercises Issuance of ordinary shares in conjunction with ESPP program

From June 30, 2019 to September 30, 2019 Issuance of ordinary shares in conjunction with vesting of restricted stock units and stock option exercises Issuance of ordinary shares in conjunction with ESPP program

from March 31, 2019 to June 30,2019 Issuance of ordinary shares in conjunction with vesting of restricted stock units and stock option exercises Issuance of ordinary shares in conjunction with ESPP program

from December 31, 2018 to March 31, 2019 Issuance of ordinary shares - public offering Issuance of ordinary shares in conjunction with vesting of restricted stock units and stock option exercises

from December 31, 2017 to December 31, 2018 Issuance of ordinary shares in conjunction with vesting of restricted stock units and stock option exercises Issuance of ordinary shares in conjunction with ESPP purchases

from December 31, 2016 to December 31, 2017 Issuance of ordinary shares in conjunction with vesting of restricted stock units and stock option exercises Issuance of ordinary shares in conjunction with ESPP purchases Issuance of ordinary shares in conjunction with PSU vesting Issuance of ordinary shares in conjunction with warrant exercises Shares repurchased

from December 31, 2015 to December 31, 2016 Issuance of ordinary shares in conjunction with vesting of restricted stock units and stock option exercises Issuance of ordinary shares in conjunction with ESPP purchases Issuance of ordinary shares in conjunction with PSU vesting Issuance of ordinary shares in conjunction with warrant exercises

from December 31, 2014 to December 31, 2015 Issuance of ordinary shares Issuance of ordinary shares in conjunction with vesting of restricted stock units and stock option exercises Issuance of ordinary shares in conjunction with inducement of convertible notes Issuance of ordinary shares in conjunction with ESPP purchases Issuance of ordinary shares in conjunction with warrant exercises

from December 31, 2013 to December 31, 2014 Issuance of ordinary shares in connection with Vidara merger Issuance of ordinary shares in conjunction with inducement of convertible notes Issuance of ordinary shares in conjunction with vesting of restricted stock units and stock option exercises Issuance of ordinary shares in conjunction with ESPP purchases Issuance of ordinary shares in conjunction with warrant exercises Treasury stock retirement Proceeds from capped call transactions Treasury stock purchase

At the effective time of the Merger on September 19, 2014 (the “Effective Time”),(i)each share of HPI’s common stock issued and outstanding was converted into one ordinary share of New Horizon; (ii) each equity plan of HPI was assumed by New Horizon and each outstanding option under HPI’s equity plans was converted into an option to acquire the number of ordinary shares of New Horizon equal to the number of common stock underlying such option immediately prior to the Effective Time at the same exercise price per share as such option of HPI, and each other stock award that was outstanding under HPI’s equity plans was converted into a right to receive, on substantially the same terms and conditions as were applicable to such equity award before the Effective Time, the number of ordinary shares of New Horizon equal to the number of shares of HPI’s common stock subject to such stock award immediately prior to the Effective Time; (iii) each warrant to acquire HPI’s common stock outstanding immediately prior to the Effective Time and not terminated as of the Effective Time was converted into a warrant to acquire, on substantially the same terms and conditions as were applicable under such warrant before the Effective Time, the number of ordinary shares of New Horizon equal to the number of shares of HPI’s common stock underlying such warrant immediately prior to the Effective Time; and (iv) the Convertible Senior Notes of HPI remained outstanding and, pursuant to a supplemental indenture entered into effective as of the Effective Time, have become convertible into the same number of ordinary shares of New Horizon at the same conversion rate in effect immediately prior to the Effective Time. Holdings retained ownership of 31,350,000 ordinary shares of New Horizon at the Effective Time.

from December 31, 2012 to December 31, 2013 Issuance of common stock in conjunction with equity financing offerings, net of underwriting fees and issuance costs Issuance of common stock in conjunction with vesting of restricted stock units and stock option exercises Issuance of common stock in conjunction with ESPP purchases Issuance of common stock in conjunction with warrant exercises

from December 31, 2011 to December 31, 2012 Issuance of common stock in conjunction with equity financing offerings, net of underwriting fees and issuance costs. Issuance of common stock in conjunction with vesting of restricted stock units Issuance of common stock in conjunction with ESPP purchases Issuance of common stock in conjuction with warrant exercises Issuance of common stock in connection with notes payable amendment

from December 31, 2010 to December 31, 2011 Conversion of convertible preferred stock to common stock Common stock issuance in public offering, net of underwriting fees and issuance costs Issuance of common stock in conjunction with the conversion of bridge notes payable Issuance of common stock in conjunction with option exercises and ESPP purchases Issuance of common stock in conjunction with warrant exercises

On July 7, 2011, the Company effected a 1-for-2.374 reverse stock split of its common stock and a proportional adjustment to the existing conversion ratios for each series of preferred stock.