Inclusion Criteria:

          -  Completion of localized therapy (prostatectomy or radiotherapy) for prostate
             adenocarcinoma (either histologically or cytologically confirmed)

          -  Biochemical (PSA) recurrence, defined as: * PSA of >= 0.2 ng/ml that has increased
             above nadir following radical prostatectomy OR * PSA increase of 2.0 ng/ml above
             post-therapy nadir after radiotherapy NOTE: PSA measured at two consecutive time
             points (separated by 4 or more weeks) is required in order to demonstrate the
             requisite increase in PSA

          -  Eastern Cooperative Oncology Group (ECOG) performance status =< 2

          -  Absolute neutrophil count >= 1,500/microliters (at baseline [pre-study])

          -  Platelets >= 80,000/microliters (at baseline [pre-study])

          -  Total bilirubin =< institutional upper limit of normal (at baseline [pre-study])

          -  Aspartate aminotransferase (AST)(serum glutamic-oxaloacetic transaminase/alanine
             aminotransferase (ALT)(serum glutamate pyruvate transaminase) =< institutional upper
             limit of normal (at baseline [pre-study])

          -  Glomerular filtration rate (GFR) >= 30 mL/min/1.73 m^2 using the Cockcroft-Gault
             formula (at baseline [pre-study])

          -  Patients with a prior or concurrent malignancy (non-prostate) whose natural history or
             treatment does not have the potential to interfere with the safety or efficacy
             assessment of the investigational regimen as determined by the treating physician are
             eligible

          -  Given that worsening of an underlying state of mental depression or suicidal ideation
             has been reported with Epidiolex, patients should be carefully screened for depression
             at baseline and if there are indications or a history of depression it is strongly
             recommended that these patients be closely followed together with behavioral health or
             psychiatric medical support. Patients with an established diagnosis of depression
             that, in the assessment of the investigator may make the administration of Epidiolex
             hazardous, should not be enrolled on this protocol

          -  Concurrent use of over-the-counter CBD oil, Marinol or marijuana is not permitted.
             Patients with a history of current over-the-counter CBD oil, Marinol or marijuana use
             for any reason are eligible only if they do the following: * Complete a one-week
             washout period prior to study initiation * Refrain from non-study related CBD oil,
             Marinol or marijuana use while on-study

          -  Ability to understand and the willingness to sign a written informed consent document

        Exclusion Criteria:

          -  History of hypersensitivity to Epidiolex (cannabidiol) or sesame seeds (one of the
             inactive ingredients in Epidiolex)

          -  Any radiological evidence of metastatic disease (determined by standard of care
             computed tomography [CT] scans of abdomen. pelvis, chest, whole body bone scan or
             Axium positron emission tomography scan). Questionable lesions on bone scan will be
             confirmed by standard of care methods such as plain X-rays or Axium positron emission
             tomography scan, if not previously performed

          -  Receipt of prior cytotoxic chemotherapy for recurrent prostate cancer

          -  Use of androgen deprivation therapy (for example, bicalutamide, flutamide, nilutamide,
             or leuprolide acetate) concurrently or within the previous 3 months.

          -  Uncontrolled intercurrent illness such as active infections. Other illnesses will be
             evaluated and eligibility status determined at the discretion of the treating
             physician and the investigator

          -  Psychiatric illness/social situations that would limit compliance with study
             requirements

          -  Concomitant use of valproate or clobazam

          -  Concurrent use of over-the-counter CBD oil, Marinol or marijuana

          -  Epidiolex is a moderate inhibitor of CYP2C19 and a moderate/strong inhibitor of
             CYP3A4, therefore concurrent use of CYP2C19 substrates is not allowed