Parkinson's disease is a disabling condition of the brain characterized by slowness of movement, shaking, stiffness, and in the later stages, loss of balance. Many of these symptoms are due to the loss of certain nerves in the brain, which results in the lack of a chemical called dopamine. Current treatments for Parkinson's are designed to increase dopamine by using levodopa (Sinemet or Madopar), which is converted in the brain into dopamine, or drugs that mimic dopamine (dopamine agonists). Although useful, these treatments do not slow the progression of the disease and can be associated with side-effects e.g. after a while levodopa use can cause involuntary movements (dyskinesia), painful leg cramps (dystonia) and a shortened response to each dose (motor fluctuations). Monoamine oxidase B (MAO-B) inhibitors such as selegiline (Eldepryl or Selgene) boost the levels of dopamine by a different mechanism, which may reduce the risk of these complications and slow disease progression. We reviewed 11 controlled trials with a total of 2514 patients that compared giving MAO-B inhibitors with not giving them in people with early Parkinson's to see if it was safe and effective. The results show that, although MAO-B inhibitors do improve symptoms of Parkinson's and delay the need for levodopa by a few months, they are too weak to have a major effect and do not seem to delay the progression of the condition. They may, however, reduce motor fluctuations although more information is needed to be certain of this. Although they can cause some side-effects, these are generally mild.