Though the primary neurotransmitter derangement associated with PD is dopamine deficiency, serotonergic, adrenergic, and cholinergic pathways may also be affected. As such, there are several comorbid conditions that frequently exist with PD. Several of these conditions are neuropsychiatric in nature. Depression is estimated to occur in up to 60% of patients, and may even be part of the disease.22 No class of antidepressant is preferred over another, but other comorbidities (insomnia, hypersomnia, and sexual dysfunction) must be taken into consideration when choosing a therapeutic agent. Some antidepressant classes have been associated with drug-induced tremor, and these drugs should be evaluated as a possible cause if symptoms worsen. Additionally, caution is warranted when using some antidepressants in conjunction with medications used to treat PD (such as the MAO inhibitors). Psychosis and dementia also commonly exist in the PD population. If antipsychotic therapy is warranted, more antidopaminergic drugs should be avoided. Preferred agents include quetiapine and clozapine. Rivastigmine is indicated for PD dementia, but because it inhibits cholinesterase, it has the potential to make PD symptoms worse by potentiating the relative overactivity of acetylcholine. Bladder dysfunction, constipation, hypotension (both medication-induced and disease-related), and falls predisposing patients to fracture are also common in the PD patient and frequently warrant intervention with drug therapy.