The primary purpose of this trial is to determine whether dose reduction of enzalutamide in patients with grade 3 fatigue and/or cognition change will lead to an improvement in symptoms while maintaining active drug levels.

Patients within 3 months of starting enzalutamide will be assessed by their oncologist as being potentially eligible for dose reduction due to the onset of moderate to severe fatigue and/or cognition change, which is assessed as being due to enzalutamide

Inclusion Criteria:

1. Patients with prostate cancer who have commenced enzalutamide within 3 months
2. Patient must have concomitant LHRH agonist or antagonist (no single agent enzalutamide)
3. Receiving enzalutamide before or after docetaxel
4. Patients may have hormone-sensitive or castrate resistant disease
5. Patients may have metastatic (M1) or non-metastatic (M0) disease
6. Onset of grade 3 or more cognition change and/or fatigue after commencement of enzalutamide considered to be due to enzalutamide

Exclusion Criteria:

1. Clinical dementia
2. Concomitant use of drugs known to impair cognition such as benzodiazepines or antihistamines.
3. Concomitant use of strong CYP3A4 and/ or CYP2C8 inducers or inhibitors.
4. Patient expected to have a change in opioid dose during the study period or have had a change 4 weeks before study entry.
5. Diagnosed with sleep apnoea
6. Brain metastases, prior seizures, drugs that significantly reduce seizure threshold.
7. Active infection or other intercurrent illness that may contribute to fatigue or cognition change within 4 weeks of study entry.

The trial involves only the male population with prostate cancer