A Study to evaluate the add-on efficacy and safety of opicapone 50 mg or an extra dose of L-DOPA 100 mg for the treatment of wearing-off in patients with PD.

Inclusion Criteria:

1. Diagnosed with idiopathic PD according to the UK Parkinson's Disease Society Brain Bank Clinical Diagnostic Criteria (2006) or according to the Movement Disorder Society (MDS) Clinical Diagnostic Criteria (2015).
2. Disease severity Stage III (H&Y staging) at ON.
3. Treated on a stable regimen for at least four weeks before screening with immediate-release L-DOPA/DDCI, three to four intakes per day, and up to maximum daily dose of 600mg L-DOPA.
4. Signs of wearing-off phenomenon with average total daily OFF-time while awake of at least 1 hour, including the early morning pre-first dose OFF (i.e. the time between wake-up and response to the first L-DOPA/DDCI dosage), despite optimal anti-PD therapy (based on Investigator's assessment).

Exclusion Criteria:

1. Non-idiopathic PD (atypical Parkinsonism, secondary [acquired or symptomatic] parkinsonism, Parkinson-plus syndrome.
2. Severe and/or unpredictable OFF periods, according to Investigator judgment.
3. Average total daily OFF-time while awake of >5 hours, including the early morning pre-first dose OFF, despite optimal anti-PD therapy (based on Investigator's assessment).
4. Treatment with prohibited medication: entacapone, tolcapone, monoamine oxidase (MAO) inhibitors (except selegiline up to 10 mg/day in oral formulation or 1.25 mg/day in buccal absorption formulation, rasagiline up to 1 mg/day or safinamide up to 100 mg/day), apomorphine or antiemetics with antidopaminergic action (except domperidone) within the last four weeks before screening.
5. Previous or planned (during the entire study duration) deep brain stimulation or stereotactic surgery (e.g. pallidotomy, thalamotomy).