The patient with Parkinson's disease often needs concomitant treatment for disorders that accompany the disease, such as depression, insomnia or constipation, or for frequent concomitant alterations such as dizziness, high blood pressure or heart disease. The many drugs that can worsen motor symptoms in Parkinson's disease must be avoided, especially if use will be prolonged. Not all drugs that induce or aggravate parkinsonism have the same potency. We describe 3 groups: 1) drugs that invariably induce or aggravate parkinsonism if taken long enough or at high enough doses; 2) drugs that only provoke parkinsonism in some individuals, and 3) drugs that interfere with the action of levodopa. Knowledge of these drugs is essential for all doctors who treat patients with Parkinson's disease.