Exhibit 99.1

GW Pharmaceuticals plc Reports Fourth Quarter and Year-End 2020 Financial Results and Operational Progress

- Total revenue of $148.2 million for the fourth quarter and $527.2 million for the full year –

- Total Epidiolex^® net product sales of $144.1 million for the fourth quarter and $510.5 million for the full year –

— Previously announced agreement to be acquired by Jazz Pharmaceuticals; transaction expected to close in Q2 2021—

London, UK and Carlsbad, CA, Feb 16, 2021 – GW Pharmaceuticals plc (Nasdaq: GWPH), a world leader in the science, development, and commercialization of cannabinoid prescription medicines, today announced financial results and operating progress for the fourth quarter and full-year ended December 31, 2020.

“We are very proud of our strong financial performance and operational progress in 2020, as Epidiolex sales increased by more than 70% during the year despite the challenges of COVID-19. We are well positioned to build on our success and continue to deliver strong growth in 2021 in both the U.S. and Europe, where we continue to make progress preparing for several commercial launches that are expected later this year,” said Justin Gover, chief executive officer of GW. “We have commenced our Phase 3 clinical program for nabiximols in the treatment of multiple sclerosis spasticity, which provides multiple opportunities for an NDA submission. Beyond nabiximols, we are advancing a diverse and robust neuroscience pipeline with several preclinical and clinical-stage pipeline candidates as part of our commitment to patients and to developing innovative medicines that address significant unmet needs. We have strong momentum and a tremendous opportunity to continue to build on our global cannabinoid leadership position as we prepare to join Jazz Pharmaceuticals and transform the lives of even more patients and families.”

FINANCIAL RESULTS

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OPERATIONAL HIGHLIGHTS

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Conference Call/Earnings Materials

Given the recently announced agreement for GW to be acquired by Jazz Pharmaceuticals, GW will no longer hold conference calls. Earnings materials are available publicly on the Investor Relations page of GW’s website at http://www.gwpharm.com. Questions may be directed to Investor Relations via e-mail at the contact information below.

About GW Pharmaceuticals plc and Greenwich Biosciences, Inc.

Founded in 1998, GW is a biopharmaceutical company focused on discovering, developing and commercializing novel therapeutics from its proprietary cannabinoid product platform in a broad range of disease areas. The company’s lead product, EPIDIOLEX^® (cannabidiol) oral solution, is commercialized in the U.S. by its U.S. subsidiary Greenwich Biosciences for the treatment of seizures associated with Lennox-Gastaut syndrome (LGS), Dravet syndrome, or tuberous sclerosis complex (TSC) in patients one year of age and older. This product has received approval in the European Union under the tradename EPIDYOLEX^® for the adjunctive treatment of seizures associated with LGS or Dravet syndrome in conjunction with clobazam in patients two years and older and is under EMA review for the treatment of TSC. The company has a deep pipeline of additional cannabinoid product candidates, in particular nabiximols, for which the company is advancing multiple late-stage clinical programs in order to seek FDA approval in the treatment of spasticity associated with multiple sclerosis and spinal cord injury. The company has additional cannabinoid product candidates in clinical trials for autism and schizophrenia.

Forward-Looking Statements

This communication contains forward-looking statements regarding Jazz Pharmaceuticals and GW Pharmaceuticals, including, but not limited to, statements related to financial performance, the timing of clinical trials, the timing and outcomes of regulatory or intellectual property decisions, the relevance of GW products commercially available and in development, the clinical benefits of Epidiolex/Epidyolex (cannabidiol) oral solution and nabiximols, and the safety profile and commercial potential of both medicines, the proposed acquisition of GW Pharmaceuticals and the anticipated timing, results and benefits thereof, including the potential for Jazz Pharmaceuticals to accelerate its growth and neuroscience leadership, and for the acquisition to provide long-term growth opportunities to create shareholder value; Jazz Pharmaceuticals’ expected financing for the

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transaction; and other statements that are not historical facts. You can generally identify forward-looking statements by the use of forward-looking terminology such as “anticipate,” “believe,” “continue,” “could,” “estimate,” “expect,” “explore,” “evaluate,” “intend,” “may,” “might,” “plan,” “potential,” “predict,” “project,” “seek,” “should,” or “will,” or the negative thereof or other variations thereon or comparable terminology. These forward-looking statements are based on each of the companies’ current plans, objectives, estimates, expectations and intentions and inherently involve significant risks and uncertainties, many of which are beyond Jazz Pharmaceuticals’ or GW Pharmaceuticals’ control. Actual results and the timing of events could differ materially from those anticipated in such forward-looking statements as a result of these risks and uncertainties, which include, without limitation, risks and uncertainties associated with: Jazz Pharmaceuticals’ and GW Pharmaceuticals’ ability to complete the acquisition on the proposed terms or on the anticipated timeline, or at all, including risks and uncertainties related to securing the necessary regulatory and shareholder approvals, the sanction of the High Court of Justice of England and Wales and satisfaction of other closing conditions to consummate the acquisition; the occurrence of any event, change or other circumstance that could give rise to the termination of the definitive transaction agreement relating to the proposed transaction; risks related to diverting the attention of GW Pharmaceuticals and Jazz Pharmaceuticals management from ongoing business operations; failure to realize the expected benefits of the acquisition; significant transaction costs and/or unknown or inestimable liabilities; the risk of shareholder litigation in connection with the proposed transaction, including resulting expense or delay; the risk that GW Pharmaceuticals’ business will not be integrated successfully or that such integration may be more difficult, time-consuming or costly than expected; Jazz Pharmaceuticals’ ability to obtain the expected financing to consummate the acquisition; risks related to future opportunities and plans for the combined company, including the uncertainty of expected future regulatory filings, financial performance and results of the combined company following completion of the acquisition; GW Pharmaceuticals’ dependence on the successful commercialization of Epidiolex/Epidyolex and the uncertain market potential of Epidiolex; pharmaceutical product development and the uncertainty of clinical success; the regulatory approval process, including the risks that GW Pharmaceuticals may be unable to submit anticipated regulatory filings on the timeframe anticipated, or at all, or that GW Pharmaceuticals may be unable to obtain regulatory approvals of any of its product candidates, including nabiximols and Epidiolex for additional indications, in a timely manner or at all; disruption from the proposed acquisition, making it more difficult to conduct business as usual or maintain relationships with customers, employees or suppliers; effects relating to the announcement of the acquisition or any further announcements or the consummation of the acquisition on the market price of Jazz Pharmaceuticals’ ordinary shares or GW Pharmaceuticals’ American depositary shares or ordinary shares; the possibility that, if Jazz Pharmaceuticals does not achieve the perceived benefits of the acquisition as rapidly or to the extent anticipated by financial analysts or investors, the market price of Jazz Pharmaceuticals’ ordinary shares could decline; potential litigation associated with the possible acquisition; regulatory initiatives and changes in tax laws; market volatility; and other risks and uncertainties affecting Jazz Pharmaceuticals and GW Pharmaceuticals, including those described from time to time under the caption “Risk Factors” and elsewhere in Jazz Pharmaceuticals’ and GW Pharmaceuticals’ Securities and Exchange Commission (SEC) filings and reports, including Jazz Pharmaceuticals’ Annual Report on Form 10-K for the year ended December 31, 2019 and Quarterly Report on Form 10-Q for the quarter ended September 30, 2020, GW Pharmaceuticals’ Annual Report on Form 10-K for the year ended December 31, 2019 and Quarterly Report on Form 10-Q for the quarter ended September 30, 2020, and future filings and reports by either company. In addition, while Jazz

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Pharmaceuticals and GW Pharmaceuticals expect the COVID-19 pandemic to continue to adversely affect their respective business operations and financial results, the extent of the impact on the combined company’s ability to generate sales of and revenues from its approved products, execute on new product launches, its clinical development and regulatory efforts, its corporate development objectives and the value of and market for its ordinary shares, will depend on future developments that are highly uncertain and cannot be predicted with confidence at this time. Moreover, other risks and uncertainties of which Jazz Pharmaceuticals or GW Pharmaceuticals are not currently aware may also affect each of the companies’ forward-looking statements and may cause actual results and the timing of events to differ materially from those anticipated. Investors are cautioned that forward-looking statements are not guarantees of future performance. The forward-looking statements made in this communication are made only as of the date hereof or as of the dates indicated in the forward-looking statements and reflect the views stated therein with respect to future events as at such dates, even if they are subsequently made available by Jazz Pharmaceuticals or GW Pharmaceuticals on their respective websites or otherwise. Neither Jazz Pharmaceuticals nor GW Pharmaceuticals undertakes any obligation to update or supplement any forward-looking statements to reflect actual results, new information, future events, changes in its expectations or other circumstances that exist after the date as of which the forward-looking statements were made.

Enquiries:

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GW PHARMACEUTICALS PLC

CONDENSED CONSOLIDATED BALANCE SHEETS

(in thousands, except share data)

(unaudited)

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GW PHARMACEUTICALS PLC

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS

(in thousands, except per share amounts)

(unaudited)

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GW PHARMACEUTICALS PLC

CONSOLIDATED STATEMENTS OF CASH FLOWS

(in thousands)

(unaudited)

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