This study looks at how a combination of medicines, Carbidopa and Levodopa, affects a condition called orthostatic hypotension (OH) in people with Multiple System Atrophy (MSA-P) and Parkinson’s Disease (PD). OH is when blood pressure drops as you stand up, causing dizziness or even fainting. Carbidopa and Levodopa are used to treat symptoms like slowness and stiffness in PD and MSA-P. However, these medicines might make OH worse. This study will check how different amounts of Carbidopa and Levodopa impact OH.

• The study is for adults aged 18 to 80 with MSA-P or PD.
• Participants should not be involved in other clinical trials.
• Pregnant or breastfeeding women cannot join the study.

If interested, participants will need to stop certain medications overnight before tests. This study helps us understand how to better manage OH in people taking these medicines.