This study explores how a medication called lithium can help people with Parkinson's disease (PD). Parkinson's is a brain disorder that affects movement. In this study, patients who have already completed a previous lithium trial will continue taking lithium for another 24 weeks. **Lithium** is a mineral often used to treat mood disorders, and it might help slow down PD. The researchers will check the effects of lithium through special brain scans called **MRIs** and blood tests, which look for changes called **biomarkers**. Biomarkers are like clues in your blood that tell doctors how a disease is behaving. - **Study Length**: 24 weeks of additional lithium therapy. - **Visits Needed**: Regular check-ups for MRI and blood tests. - **Risks**: Possible side effects from lithium; must not have heart, brain, or mental health issues. Only patients who have completed specific previous studies at the University at Buffalo can join. Participants will need a stable health condition and women must take certain precautions if they can have children. This study hopes to find the best lithium level to improve PD symptoms.

**Eligibility:** Must have finished a prior study, stable health, follow guidelines.