Third Cancer Pain Trial Commences

THIRD PHASE III SATIVEX® CANCER PAIN TRIAL COMMENCES

Porton Down, UK; 22 May 2012: GW Pharmaceuticals plc (AIM: GWP) today announced the initiation of a third Phase III Sativex® clinical trial in the treatment of pain in patients with advanced cancer, who experience inadequate analgesia during optimized chronic opioid therapy.

GW's cancer pain clinical programme is being wholly funded by Otsuka Pharmaceutical Co. Ltd, which has licensed the US commercialisation rights to this product. The trials are designed to obtain approval in this indication from the Food & Drug Administration (FDA) in the US, and these data will also be used by GW for future regulatory applications in this indication in Europe and around the world.

GW has previously announced the start of two pivotal 380 patient Phase III cancer pain studies, both of which are proceeding on track and are expected to complete recruitment around the end of 2013. Regulatory filings are intended to be made upon completion of these two studies. This newly commenced third Phase III trial is a supportive study intended to provide, as needed, supplementary data to that generated in the first two studies.

The third Phase III trial differs in design from the first two studies, employing an "enriched study design" akin to that which was successfully employed in the MS spasticity trials programme. The study involves exposing patients to Sativex in a single blind phase of two weeks duration ("Phase A"), following which responders will be randomised either to stay on Sativex or switch to placebo in a double blind phase for a five week treatment period ("Phase B"). The primary efficacy analysis will be the mean change from baseline in Phase B as measured using a 0-10 Numeric Rating Scale (NRS). The study will aim to recruit 540 patients into Phase A and target 216 patients to enter Phase B.

Prior to commencing the Phase III programme, GW completed two Phase II studies with positive results including over 500 patients in total. The most recent Phase IIb study was published last month in the Journal of Pain (1), the official journal of the American Pain Society.

Dr. Stephen Wright, GW's R&D Director, said, "We are pleased to have commenced this planned third Phase III study evaluating the efficacy and safety of Sativex as a potential treatment for cancer pain. This clinical development programme, being performed in partnership with Otsuka, is the largest ever undertaken by GW. With significant numbers of patients with advanced cancer around the world suffering pain in spite of treatment with opioid therapy, Sativex has the potential to meet a very significant unmet need."

Sativex is licensed to Otsuka in the US, Almirall S.A. in Europe (excluding the United Kingdom) and Mexico, Bayer HealthCare AG in the UK and Canada, Novartis Pharma AG in Australasia, Asia (excluding Japan and China), Middle East (excluding Israel) and Africa, and Neopharm Group in Israel.

(1) http://www.jpain.org/article/S1526-5900(12)00019-3/abstract

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Notes to Editors:

Sativex® in Multiple Sclerosis

Sativex® is currently available as a prescription medicine for the treatment of spasticity associated with multiple sclerosis in the UK, Spain, Germany, Denmark, Canada and New Zealand. Sativex is also approved/recommended for approval in Italy, Austria, Sweden, Czech Republic, Belgium, Finland, Iceland, Ireland, Luxembourg, the Netherlands, Norway, Poland, Portugal and Slovakia.

Sativex® is indicated as treatment for symptom improvement in patients with moderate to severe spasticity due to multiple sclerosis (MS) who have not responded adequately to other anti-spasticity medication and who demonstrate clinically significant improvement in spasticity related symptoms during an initial trial of therapy.1

About GW

GW was founded in 1998 and listed on the AIM, a market of the London Stock Exchange, in June 2001. Operating under license from the UK Home Office, the company researches and develops cannabinoid pharmaceutical products for patients who suffer from a range of serious ailments, in particular multiple sclerosis and cancer pain. GW has assembled a large in-house scientific team with expertise in cannabinoid science as well as experience in the development of both plant-based prescription pharmaceutical products and medicines containing controlled substances. GW occupies a world leading position in cannabinoids and has developed an extensive international network of the most prominent scientists in the field. For further information, please visit www.gwpharm.com

This news release may contain forward-looking statements that reflect GWs current expectations regarding future events, including development and regulatory clearance of the GW's products. Forward-looking statements involve risks and uncertainties. Actual events could differ materially from those projected herein and depend on a number of factors, including (inter alia), the success of the GW's research strategies, the applicability of the discoveries made therein, the successful and timely completion of uncertainties related to the regulatory process, and the acceptance of Sativex® and other products by consumer and medical professionals.

1 Sativex® Summary of Product Characteristics, 2011.