Inclusion criteria:
1. The participant is 16 to 40 years of age, willing and able to provide written informed consent/assent.
2. The participant is currently being treated with an antipsychotic for at least 3 weeks but no longer than 16 weeks.
3. The participant should be receiving at least the minimum dose of this antipsychotic for FEP according to modified Maudsley guidelines, as listed in Appendix B. The clinician should judge the participant to be a non-responder to this treatment and that increasing the dose further is unacceptable to the clinician and/or participant.
4. The compliance score associated with this antipsychotic medication is 4 or more on the Clinician Rating Scale (CRS).
5. The participant meets DSM-5 criteria for schizophrenia, schizoaffective disorder or schizophreniform disorder, as confirmed through the SCID-5-RV.
6. The participant does not meet modified Andreasen criteria for symptomatic remission (time requirement does not apply).
7. Participants of childbearing potential (\*) must be willing to ensure that they use highly effective contraception during the trial and as per the requirements in the protocol\*\*.
8. In the Investigator's opinion, is able and willing to comply with all trial requirements.
9. Willing to allow their General Practitioner and/or consultant, if required by local guidelines/regulations, to be notified of participation in the trial.
10. For participants who take part in the optional MRI scans: they must be eligible for MRI scanning as per local requirements, for example concerning e.g. implants, braces etc.
There is inadequate information on the effects of cannabidiol on the foetus in humans. Participants of childbearing potential\* should use a highly effective method of contraception\*\* for the duration of the trial and for 3 months after the last time the trial intervention was used.
\*A person is considered of childbearing potential, i.e. fertile, following menarche and until becoming post-menopausal unless permanently sterile. Permanent sterilisation methods include hysterectomy, bilateral salpingectomy and bilateral oophorectomy. A postmenopausal state is defined as no menses for 12 months without an alternative medical cause.
\*\* Methods that can achieve a failure rate of less than 1% per year when used consistently and correctly are considered as highly effective birth control methods. Such methods include: 1) combined (oestrogen and progestogen containing) hormonal contraception associated with inhibition of ovulation (oral; intravaginal; transdermal); 2) progestogen-only hormonal contraception associated with inhibition of ovulation (oral; injectable; implantable); 3) intrauterine device (IUD); 4) intrauterine hormone-releasing system (IUS); 5) bilateral tubal occlusion; 6) vasectomised partner; 7) sexual abstinence (abstinence should only be used as a contraceptive method if it is in line with the participants' usual and preferred lifestyle).
Periodic abstinence (calendar, symptothermal, post-ovulation methods) is not an acceptable method of contraception. The participant agrees to use an acceptable method of contraception\* for the duration of the study and for 3 months after any study drug administration, unless surgically sterile or postmenopausal (no menses for 12 months without an alternative medical cause).
The age range for eligibility has been applied as this corresponds to the usual age range for first episode psychosis; individual cases outside of this age range may have a different aetiology and/or prognosis which could impact on the study outcomes.
Exclusion criteria:
1. Having been previously treated with a different antipsychotic (to the current one) at an adequate dose\* for 4 weeks or longer.
2. Current or previous treatment with clozapine and/or current treatment with sodium valproate, valproate semisodium, or clobazam. In cases of current use of these medicines, participation is only permitted if they can and will be discontinued or switched to a suitable alternative medication prior to randomisation.
3. Hypersensitivity to the active substance, sesame oil, sesame seed or any of the excipients of the intervention.
4. Known hepatic insufficiency and/or transaminase elevations levels exceeding the upper limit of normal 2 times or more and bilirubin greater than 1.5 times the upper limit of normal.
5. Previous neurosurgery or neurological disorder, including epilepsy, which may affect the study procedures\*\*.
6. IQ \<70.
7. Pregnancy or breastfeeding.
8. The patient has a current diagnosis of 'Substance or medication induced psychotic disorder' or 'Psychotic disorder due to another medical condition' as determined through the SCID-5-RV.
9. Current active suicidal ideation within the last 2 weeks, defined as a score of 1 or higher on CDSS question 8, followed by an assessment by the treating clinician who determines it is not safe for the patient to participate in the study\*\*\*.
10. Meeting DSM-V criteria for substance use disorder, with the exception of nicotine use disorder (mild, moderate, and severe allowed). Mild cannabis use disorder is allowed (i.e. can meet up to but no more than 3 criteria on the SCID) as long as the subject patient has not consumed cannabis on average more than three times a week in the past 30 days. Mild alcohol use disorder is also allowed.
11. Patient has participated in another clinical trial in which they received an experimental or investigational drug or agent within 3 months before Visit 0. Patients who have participated in Type A studies (e.g. trials of standard and within-label treatments including antipsychotic medication) or non CTIMP studies (e.g. studies of exercise therapy) must have completed the intervention but may be included if permitted by the protocol of the other trial.
12. The participant refuses any mandatory safety checks during the trial, specifically, refusal of: urine pregnancy test (those of child-bearing potential only); safety blood test; reporting of adverse events; and assessment of suicidality.
13. Any other significant disease or disorder which, in the opinion of the Investigator, may either put the participants at risk because of participation in the trial, or may influence the result of the trial, or the participant's ability to participate in the trial.
* Adequate doses are provided in Appendix B. \*\*Minor neurological disorders such as migraine, other minor headache disorders, sleep disorders or nerve palsies which are unlikely to affect study outcomes including neuroimaging measures are permitted.
* The decision to include the patient is at clinician's discretion. The clinician can conclude that it is safe for the patient to participate after the patient is evaluated, in which case this exclusion criterion does not apply and the patient can participate. Either way, the treating clinician needs to record his/her evaluation of suicidal risk in the source documentation or medical file, including his/her considerations, and notify the site PI of the decision.
