Hallucinations are a side effect in about 30 percent of people taking Parkinson's drugs -- particularly dopamine agonists -- and they usually occur only after many years. The problem is more likely to arise in patients with dementia. A symptom of psychosis, the hallucinations are often benign at first, involving visions of children, strangers, or animals around the home. Usually, a Parkinson's patient realizes the episodes aren't real but may not tell anybody about them for fear that he's losing his mind.

Parkinson's drugs may also trigger serious psychosis, in which people can't tell what's real and what's not. About 5 to 10 percent of people with Parkinson's disease get caught up in delusions (false or illogical beliefs) and paranoia -- such as becoming convinced that others are spying on them. The most common delusion, says Friedman, is that the patient's spouse is having an extramarital affair. Such distressing psychotic symptoms can make the patient extremely difficult to care for, and they're a major factor leading to placements in nursing homes.

To treat hallucinations and delusions, doctors switch or try to reduce the Parkinson's medicine -- but the catch-22 is that the pills are crucial for controlling the patient's motor problems. Physicians may prescribe atypical antipsychotic drugs such as Seroquel (quetiapine) or Clozaril (clozapine).