Toxicology – Levodopa/Carbidopa Toxicity

Source

Levodopa and carbidopa are commonly prescribed together for Parkinson disease. Levodopa is a precursor of dopamine that can cross the blood–brain barrier, while carbidopa reduces its conversion to dopamine outside the central nervous system.

Typical Presentation

Patients with overdose may appear markedly agitated or confused and can develop hallucinations or bizarre behavior. The presentation may resemble a sympathomimetic or anticholinergic toxidrome, particularly in older adults.

Clinical Features

Acute toxicity commonly causes tachycardia, hypertension, dilated pupils, psychomotor agitation, delusions, and hallucinations. Nausea, vomiting, urinary retention, and involuntary choreoathetoid movements may also occur. Blood pressure can fluctuate, with early hypertension followed by hypotension or orthostatic hypotension. Sustained-release formulations may produce delayed or recurrent symptoms for up to 48 hours.

Mechanism of Action

Levodopa crosses into the brain and is converted into dopamine. Carbidopa inhibits peripheral dopamine formation, allowing more levodopa to reach the CNS. In overdose, excessive dopamine and catecholamine activity produces both neurological and cardiovascular stimulation.

Management

Treatment is mainly supportive, with cardiac and blood pressure monitoring. Activated charcoal may be considered after a recent significant ingestion. Patients who ingest large amounts of sustained-release formulations may require prolonged observation because symptoms can recur after an initial period of improvement.

Key Points

• Mydriasis, agitation, and hallucinations in a patient taking dopaminergic medications should raise concern for overdose.
• Hypertension may later transition to hypotension, so antihypertensive treatment should be used cautiously.
• Sustained-release products can cause delayed recurrent toxicity.
• Abrupt reduction or withdrawal of levodopa/carbidopa during therapeutic use can precipitate a neuroleptic malignant syndrome–like reaction.