The main use of selegiline is in the treatment of Parkinson's disease. It can be used on its own or in a combination with another agent, most often L-DOPA. For the newly diagnosed Parkinson's patients, selegiline appears to slow the progression of the disease. It delays the time point when the L-DOPA (levodopa) treatment becomes necessary from 10-12 to 18 months. The idea behind adding selegiline to levodopa is to decrease the dose of levodopa and thus reduce the motor complications of levodopa therapy. Comparisons of patients on levodopa + placebo vs levodopa + selegiline showed that selegiline allowed reduction of the levodopa dose by about 40%. Selegiline + levodopa also extended the time until the levodopa dose had to be increased from 2.6 to 4.9 years. As a result there were fewer motor complications in selegiline groups. In one trial, selegiline + levodopa completely halted the progress of Parkinson's disease over 14 months, while in the placebo + levodopa group the deterioration of the patients' condition continued. However, the interpretation of this trial as proving neuroprotective action of selegiline has been questioned
As of February 28, 2006, selegiline has also been approved by the Food and Drug Administration (FDA) to treat major depression using a transdermal patch (Emsam Patch). Selegiline is also used (at extremely high dosages relative to humans) in veterinary medicine to treat the symptoms of Cushing's disease and so-called "cognitive dysfunction" in dogs. As of June 26, 2006, a selegiline transdermal patch is being tested for its effectiveness in treating ADHD.
Several clinical studies are currently underway to evaluate selegiline's effectiveness in helping people stop smoking tobacco or marijuana.
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