Clinical Trial to Observe the Effects of Tamoxifen on Testosterone Recovery in Medically Castrated Prostate Cancer Patients

Inclusion Criteria:

* At least 18 years of age;
* Ability to understand the purposes and risks of the trial and has signed a written informed consent form. Have a diagnosis of prostate cancer;
* Patient received ADT for a duration of either 6 or 18-36 months as part of the curative intent treatment. Curative intent prostate cancer patients who completed ADT and have had no further ADT for the length of the last ADT injection depot formulation (e.g., if the last ADT injection depot formulation is for 3 months, the patient must have no ADT for 3 months after last injection)
* Have effectively castrated testosterone (\< 1.7 nmol/L \[50 ng/dL\]) within 6 weeks of enrollment;
* ECOG Performance status 0-2

Exclusion Criteria:

* Harbouring certain CYP2D6 alleles (i.e. CYP2D6\*4) or from the chronic use of a CYP2D6 inhibitor(s);
* History of blood clots (venous thromboembolism or pulmonary embolism);
* History of stroke or transient ischemic attack (TIA);
* Reduced liver function within last 120 days prior to enrolment, defined as follows:

  1. Total Bilirubin: 1.5 \> upper limit of normal (ULN) (For Gilbert's syndrome, if total bilirubin is \<1.5 x ULN, measure direct and indirect bilirubin. If direct bilirubin is greater than 1.5 x ULN, participant is ineligible;
  2. AST(SGOT) and ALT(SGPT): \> 2.5x ULN;
  3. Or other liver disease as deemed ineligible by the investigator
* Baseline QT/QTc \> 500ms;
* Active therapy with selective serotonin reuptake inhibitor (SSRI) antidepressants (e.g. paroxetine, a known CYP2D6 inhibitor);
* Active therapy with coumarin-type anticoagulants;
* Active therapy with cytotoxic agents;
* Active therapy with aromatase inhibitors;
* Other invasive malignancy within the last 5 years, other than squamous or basal cell carcinoma of the skin;
* Treatment with a non-approved or experimental drug during the 3 months before informed consent;
* Patients known to have one of the following hereditary illnesses; galactose- intolerance, Lapp lactase deficiency or glucose-galactose malabsorption;
* Any other significant concomitant disease or condition that could interfere with the conduct of the study, or that would, in the opinion of the investigator, pose an unacceptable risk to the participant in this trial;