A randomized, double-blind, placebo-controlled study to investigate the efficacy and safety of cannabidiol (GWP42003-P; CBD) as adjunctive treatment for seizures associated with Lennox- Gastaut syndrome in children and adults

* Patient and/or parent(s)/legal representative must be willing and able to give informed assent/consent for participation in the study.;* Patient and their caregiver must be willing and able (in the investigator*s opinion) to comply with all study requirements.;* Patient must be male or female aged between two and 55 years (inclusive).;* Patient must have a clinical diagnosis of of LGS. This includes written documentation of having met electroencephalogram (EEG) diagnostic criteria during the patient*s history and evidence of more than one type of generalized seizure, including drop seizures (atonic, tonic or tonic-clonic), for at least six months. Care should be taken not to include benign myoclonic epilepsy of infancy, atypical benign partial epilepsy (pseudo-Lennox syndrome), or continuous spike-waves of slow sleep (CSWS);* Patients who have a history of slow (<3.0 Hz) spike-and-wave pattern in an EEG prior to the enrollment into the baseline period.;* Patients must have at least two drop seizures each week during the 28-day baseline period.;* Patients should be refractory; that is having documented failures on more than one antiepileptic drug (AED).;* Patient must be taking one or more AEDs at a dose which has been stable for at least four weeks prior to screening.;* All medications or interventions for epilepsy (including ketogenic diet and vagus nerve stimulation [VNS]) must have been stable for four weeks prior to screening and patient is willing to maintain a stable regimen throughout the study. The ketogenic diet and VNS treatments are not counted as an AED.;* Patient and/or parent(s)/legal representative is willing to allow his or her primary care practitioner and consultant to be notified of participation in the study.

* Etiology of patient's seizures is a progressive neurologic disease. Patients with tuberous sclerosis will not be excluded from study participation, unless there is a progressive tumor.;* Patient has had an anoxic episode requiring resuscitation within six months of screening.;* Patient has clinically significant unstable medical conditions other than epilepsy.;* Patient has had clinically relevant symptoms or a clinically significant illness in the four weeks prior to screening or randomization, other than epilepsy.;* Patient has clinically significant abnormal laboratory values, in the investigator*s opinion, at screening or randomization.;* Patient has clinically relevant abnormalities in the ECG measured at screening or randomization.;* Patient has any concurrent cardiovascular conditions, which will, in the investigators opinion, interfere with the ability to assess their ECGs.;* Patient has a history or presence of alcohol or substance abuse within the last two years prior to the study or daily consumption of five or more alcohol-containing beverages.;* Patient is currently using or has in the past used recreational or medicinal cannabis, or synthetic cannabinoid based medications (including Sativex®) within the three months prior to study entry. ;* Patient is unwilling to abstain from using recreational or medicinal cannabis, or synthetic cannabinoid based medications (including Sativex) during the study.;* Patient has a history of symptoms (e.g., dizziness, light-headedness, blurred vision, palpitations, weakness, syncope) related to a drop in blood pressure due to postural changes.;* Patient has any known or suspected hypersensitivity to cannabinoids or any of the excipients of the IMP, such as sesame oil.;* Female patient is of child bearing potential and must have a negative pregnancy test and be willing and able to use a reliable method of contraception throughout the trial and for three months after last dose. Male patient*s partner is of child bearing potential; unless willing to ensure that they or their partner use a highly effective method of contraception. In the context of this trial, a highly effective method is defined as those which result in low failure rate (i.e., less than 1% per year) when used consistently and correctly such as: combined or progesterone only oral contraceptives, intrauterine device, intrauterine hormone-releasing system, bilateral tubal occlusion, vasectomized partner or sexual abstinence;* Female patient who is pregnant, lactating or planning pregnancy during the course of the study and for three months thereafter.;* Patient has been part of a clinical trial involving another IMP in the previous six months.;* Any other significant disease or disorder which, in the opinion of the investigator, may either put the patient at risk because of participation in the study, may influence the result of the study, or affect the patient*s ability to participate in the study.

* Patient has significantly impaired hepatic function at screening (Visit 1) or randomization (Visit 2) (Alanine aminotransferase [ALT] >5 x upper limit of normal [ULN] and total bilirubin [TBL] >2 x ULN) OR the ALT or Aspartate aminotransferase (AST) >3 x ULN and (TBL >2 x ULN or international normalized ratio [INR] >1.5). This criterion can only be confirmed once the laboratory results are available; patients randomized into the study who are later found to meet this criterion must be withdrawn from the study.;* Following a physical examination, the patient has any abnormalities that, in the opinion of the investigator, would prevent the patient from safe participation in the study.;* Any history of suicidal behavior or any suicidal ideation of type four or five on the C-SSRS in the last month or at screening.;* Patient is unwilling to abstain from donation of blood during the study.;* There are plans for the patient to travel outside their country of residence during the study.;* Patient has previously been randomized into this study.;* Patient is taking more than four concurrent AEDs.;* Patient has taken corticotropins in the six months prior to screening.;* Patient is currently taking long-term systemic steroids (excluding inhaled medication for asthma treatment) or any other daily medication known to exacerbate epilepsy. An exception will be made for prophylactic medication, for example, idiopathic nephrotic syndrome or asthma. ;* Patient is taking felbamate, and they have been taking it for less than one year prior to screening.