Carbidopa-levodopa, the most widely prescribed medication for Parkinson’s disease, causes side effects ranging from mild nausea to involuntary movements that develop over years of use. About 65% of people on levodopa develop some form of involuntary movement within five years. Most side effects are manageable, but knowing what to expect at each stage of treatment helps you recognize problems early and respond effectively.
Digestive Side Effects
Nausea is the most common complaint when starting carbidopa-levodopa, and it’s often the reason people struggle with the medication early on. The carbidopa component exists specifically to reduce this problem by blocking levodopa from converting to dopamine outside the brain, but it doesn’t eliminate nausea entirely. Other digestive side effects include loss of appetite, vomiting, diarrhea, constipation, abdominal pain, and dry mouth.
A few practical strategies can make a real difference. Taking the medication with a full glass of water helps it dissolve and move through your digestive system. If nausea persists, eating a small low-protein snack like crackers or drinking juice alongside your dose can settle your stomach. The key is keeping the snack low in protein, because protein-rich foods compete with levodopa for absorption in your gut, potentially reducing how well the drug works. Timing matters: take carbidopa-levodopa 30 minutes before meals or one to two hours after. If nausea still doesn’t resolve, your doctor may increase the proportion of carbidopa in your prescription.
Dizziness and Blood Pressure Drops
Levodopa lowers blood pressure in both the lying and standing positions, which can cause lightheadedness or fainting when you stand up. This effect, called orthostatic hypotension, happens because the medication impairs your body’s normal reflex for adjusting blood pressure when you change position. Research from Neurology found that levodopa caused greater blood pressure drops at 7 and 10 minutes of standing, with reductions averaging around 7 to 8 points on systolic blood pressure compared to being off the medication.
What makes this tricky is that levodopa can unmask blood pressure instability even in people who didn’t previously show signs of it. If you notice dizziness when getting out of bed or standing from a chair, rising slowly and pausing before walking can help. Staying hydrated also supports blood pressure stability.
Involuntary Movements (Dyskinesia)
Dyskinesia is one of the most significant long-term side effects of levodopa therapy. These are involuntary, uncontrolled movements that can affect the mouth, tongue, face, head, neck, arms, or legs. They typically appear not in the first weeks of treatment but after months or years. A population-based study in Olmsted County, Minnesota found that 65.5% of patients developed dyskinesia within five years of starting levodopa.
Dyskinesia usually occurs when levodopa levels in the brain peak after a dose. It reflects the brain’s changing sensitivity to dopamine over time, not a sign that the medication has stopped working. Managing dyskinesia often involves adjusting the dose, changing how frequently you take the medication, or adding other treatments to smooth out dopamine levels throughout the day.
The “Wearing Off” Effect
Over time, many people notice that their Parkinson’s symptoms return before their next dose is due. This is called wearing off, and it reflects the brain’s decreasing ability to store and steadily release dopamine from each dose. Wearing off can show up as a return of tremor, stiffness, or slowness, but it also includes non-motor symptoms like anxiety, fatigue, or mood changes.
Wearing off can come on gradually or hit suddenly and unpredictably. Morning “off” periods, before the first dose of the day kicks in, are especially common. Keeping a log of when symptoms return relative to your dosing schedule gives your doctor useful information for adjusting your regimen.
Hallucinations and Psychiatric Effects
Levodopa can cause hallucinations, most commonly visual ones like seeing people, animals, or objects that aren’t there. Prevalence estimates in Parkinson’s patients range from 5% to 50%, depending on the population studied and how hallucinations are defined. Older age, exposure to multiple medications, and cognitive impairment all raise the risk. These effects are more common in later stages of treatment rather than early on.
Other psychiatric side effects include vivid nightmares, confusion, forgetfulness, depression, nervousness, and difficulty sleeping. Some people develop impulse control disorders, which are compulsive behaviors like excessive gambling, shopping, eating, or repetitive purposeless activities (sometimes called “punding”). Impulse control disorders affect up to 20% of people with Parkinson’s disease on dopaminergic medications, though they’re more commonly associated with a related class of drugs called dopamine agonists than with levodopa itself.
Vitamin B6 Depletion and Seizure Risk
In March 2026, the FDA required a new warning on all carbidopa-levodopa products about the risk of vitamin B6 deficiency. The mechanism is twofold: levodopa’s conversion to dopamine uses up vitamin B6, and carbidopa binds to the active form of B6, creating additional functional loss. Higher doses carry greater risk. In the cases the FDA reviewed, all involved levodopa doses exceeding 1,000 mg daily, and doses above 1,500 mg were associated with a shorter time to developing deficiency.
The most serious consequence of severe B6 depletion is seizures, and these seizures don’t respond to standard anti-seizure medications. They resolve after B6 is given. Some anti-seizure drugs can actually worsen B6 deficiency, compounding the problem. Doctors are now directed to check B6 levels before starting treatment and periodically during therapy, supplementing as needed.
Protein and Drug Absorption
Levodopa and dietary protein use the same transport system to cross the lining of your gut. When you eat a protein-rich meal close to your dose, the amino acids from that meal compete directly with levodopa for absorption, reducing how much medication reaches your brain. This can make it seem like your medication has stopped working when the real issue is timing.
The standard advice is to separate protein intake from your doses. But as researchers at the University of Florida’s Fixel Institute point out, this isn’t one-size-fits-all. Factors like your age, disease severity, genetics, and individual digestive function all affect how much protein interferes with absorption. Working with a dietitian to create a meal plan that maintains good nutrition while optimizing medication timing is more effective than following generic rules.
Who Should Not Take It
Carbidopa-levodopa is contraindicated in people with narrow-angle (closed-angle) glaucoma. People with the more common open-angle glaucoma can generally use the medication safely as long as their glaucoma is being treated. If you’re unsure which type of glaucoma you have, your eye doctor can clarify this before you start treatment.