Patients with Parkinson's disease have low levels of the neurotransmitter dopamine. Dopamine is responsible for motor function and normal physical activity. Patients with Parkinson's disease typically suffer from tremors, rigid muscles, stooped postures, and walk with a shuffle. The drug levodopa acts as a replacement for dopamine and has been has been used effectively for over 30 years as treatment for Parkinsons disease. Because of its effectiveness, levodopa has been used to distinguish Parkinson's disease from other conditions that may resemble Parkinson's disease.