6. Interactions: levidopa

The vitamin pyridoxine (B6) increases the peripheral breakdown of levodopa and diminishes its effectiveness. Concomitant administration of levodopa and monoamine oxidase inhibitors (MAOIs), such as phenelzine, can produce a hypertensive crisis caused by enhanced catecholamine production. Therefore, caution is required when they are used simultaneously. In many psychotic patients, levodopa exacerbates symptoms, possibly through the buildup of central catecholamines. In patients with glaucoma, the drug can cause an increase in intraocular pressure. Cardiac patients should be carefully monitored because of the possible development of cardiac arrhythmias. Antipsychotic drugs are generally contraindicated in parkinsonian patients, because these potently block dopamine receptors and produce a parkinsonian syndrome themselves. However low doses of certain “atypical” antipsychotic agents are sometimes used to treat levodopa-induced psychiatric symptoms.