This study will recruit schizophrenia patients who use cannabis recreationally. Each participant will attend the laboratory on three occasions: an initial visit to check that they are safe to join the study and two days of testing.

Participants will be administered, in a randomized order, a pre-treatment with either CBD (1000mg) orally or a matching placebo. On both experiments, participants will then inhale cannabis containing THC. The THC administration will follow a standardised inhalation procedure using a medical-grade vaporizer device.

Participants will complete a series of tasks measuring cognition, psychosis, anxiety and other subjective experiences.

The study will be carried out at the NIHR-Wellcome Trust Clinical Research Facility at King's College Hospital.

Inclusion criteria:

• Age 18-65 years.
• Clinical diagnosis of schizophrenia (i.e. documented as such in the patient's clinical records and satisfying ICD-10 criteria for F20)
• Clinically stable for at least three months (since discharge from hospital, home treatment team, or prior clinical deterioration, and with agreement from the patient's responsible clinician)
• Regular (at least weekly) cannabis use for the past 3 months or more
• Evidence from either clinicians or from the patient that cannabis use exacerbates their symptoms or increases their risk of relapse
• Treatment with regular doses of antipsychotic medication for at least 1 month, confirmed by a blood test at the baseline visit, and with the participant agreeing to be maintained at a stable dose over the course of the experiment
• The participant agrees to abstain from cannabis use for at least 24hours prior to study visits
• The participant is willing to have an intravenous cannula inserted to collect blood samples on experimental visits
• Sufficiently fluent English
• Providing written informed consent

Exclusion criteria:

• Extreme cannabis use: participant is estimate to be using over 1gram of cannabis/day
• Dependence on alcohol or illicit substances other than cannabis as defined by ICD-10
• Pregnancy (current or planned) or breastfeeding
• Physical health disorder or another mental health disorder that the study psychiatrist judges may influence the patient's ability to tolerate the procedure, or that may alter the results of the study.
• Taken part in any drug study within the last 3 months or taking part in another study over the course of the trial
• Drug sensitivity/allergy to cannabis or Lorazepam
• Unlikely to be able to complete the study sessions for any reason, as judged by the study psychiatrist

Additional criteria which must be met on experimental visits:

• Negative alcohol breath test
• Negative urine drug screen (apart from cannabis and prescribed medication)
• Negative urine pregnancy test
• Stable mental state as judged by the study psychiatrist