The FDA now requires a seizure warning on a common Parkinson’s drug millions take

Since March 2026, drugmakers have been required to add a new warning to carbidopa/levodopa, the standard combination therapy for Parkinson’s disease symptoms, after the Food and Drug Administration linked the medication to a rare but serious form of vitamin B6 deficiency that can trigger seizures. The requirement covers widely prescribed brand names including Sinemet, Rytary and Duopa, along with their generic equivalents, and applies to a drug combination that remains the most effective treatment available for the tremors and stiffness caused by Parkinson’s disease.

What the FDA’s Safety Review Found

The FDA’s drug safety communication, issued March 20, 2026, followed a review that identified 14 cases of seizures tied to vitamin B6 deficiency in patients taking carbidopa/levodopa, most drawn from reports submitted to the agency’s Adverse Event Reporting System and one identified in published medical literature. Every one of the reviewed cases involved levodopa doses above 1,000 milligrams a day, and patients on doses above 1,500 milligrams tended to develop the deficiency faster, with latency periods before diagnosis ranging from 23 to 132 months of treatment. Two of the 14 patients died, both with documented low vitamin B6 levels and seizures that had proven difficult to control.

Why the Seizures Do Not Respond to Ordinary Treatment

The mechanism behind the reaction is specific to how these drugs are processed by the body. Levodopa is converted into dopamine using vitamin B6 as a cofactor, and carbidopa separately binds to the active form of the vitamin, compounding the depletion over time. According to the FDA’s published alert, the resulting seizures typically present as focal seizures that spread into full-body convulsions, consistent with a recognized pattern called vitamin B6-dependent epilepsy, and in some cases progressed to status epilepticus, a medical emergency involving prolonged or repeated seizures. Standard anti-seizure medications did not stop these episodes in the reviewed cases; of nine patients who instead received vitamin B6 supplementation, all nine saw their seizures resolve, even though most had already failed multiple traditional anti-seizure drugs.

Which Products the Warning Covers

The new labeling requirement applies broadly across the carbidopa/levodopa product family, including oral tablets sold as Sinemet and Sinemet CR, the extended-release capsule Rytary, the newer twice-daily formulation Crexont, the intestinal-gel suspension Duopa, the combination product Stalevo that adds entacapone, and the subcutaneous infusion product Vyalev. The FDA’s review found no confirmed cases tied specifically to the entacapone combination or the injectable formulation, which the agency attributes to their more recent approval dates and lower overall usage, but it still required the warning across the entire product class because vitamin B6 depletion showed up in clinical trial data supporting the injectable drug’s original approval, suggesting the underlying risk is not confined to the older oral formulations. The FDA also noted that the absence of reported cases for those two products may simply reflect their smaller patient populations and more recent market entry rather than a genuine difference in biological risk, which is why the agency chose to apply the same warning language across every carbidopa/levodopa product rather than carving out exceptions.

What the Warning Asks Doctors and Patients to Do

The label change directs prescribers to check a patient’s vitamin B6 levels before starting carbidopa/levodopa therapy and periodically afterward, particularly in patients on higher doses, and to supplement with vitamin B6 when levels run low. Patients and caregivers are advised to watch for early symptoms beyond seizures, including unexplained depression, confusion, inflammation of the lips, tongue or skin, and nerve symptoms such as numbness, tingling or muscle weakness, and to raise those symptoms with a health care professional rather than adjusting the medication independently. Parkinson’s disease itself remains a progressive neurological condition with no cure, so the FDA’s guidance is aimed at monitoring for the newly identified deficiency rather than discouraging use of a drug combination that remains central to managing the disease’s motor symptoms.

A Warning Built on Decades of Use

Carbidopa/levodopa has been a mainstay of Parkinson’s treatment for decades, which is part of why the newly identified risk took so long to surface: the seizure cases reviewed by the FDA developed only after months or years of continuous, often escalating dosing, making the connection to vitamin B6 easy to miss without a formal safety review pulling scattered reports together. The agency’s review also found that seizures resolved with B6 supplementation even in patients who had already been placed on multiple anti-seizure drugs without success, a detail that distinguishes this reaction from more familiar drug-induced seizures and explains why the new labeling specifically calls out B6 replacement, rather than standard anti-epileptic therapy, as the recommended response. The agency has also asked health care professionals and patients to continue reporting any new seizures or symptoms of vitamin B6 deficiency through its MedWatch Safety Information and Adverse Event Reporting Program, noting that the 14 cases identified so far likely undercount the true total since the review relied heavily on voluntary postmarketing reports rather than a systematic screening of every patient on the therapy.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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