Because the motor manifestations of Parkinson's disease are due to an inadequate production of dopamine by a certain area of the brain, the substantia nigra, it should be no surprise that the most effective medication provides the brain with an abundant supply of the precursor it uses to make dopamine. This medication is known generically as carbidopa/levodopa, but has several brand names now, including the shorter acting SinemetTM and the longer acting RytariTM. The brain uses the levodopa in carbidopa/levodopa to make dopamine. The carbidopa is included to keep the body from converting levodopa to dopamine outside the brain, where it would cause severe nausea and possibly irregular heartbeats.
Carbidopa/levodopa also has the best side effect profile of the medications used for the motor manifestations of Parkinson's. However, some people may develop some nausea while taking it, especially at higher doses, and they may require the use of a different medication or additional strategies to reduce the nausea. Additionally, a small percentage of people with Parkinson's who take carbidopa/levodopa will experience a fall in blood pressure when they stand, a phenomena called orthostatic hypotension, and this can cause dizziness on standing.
The dopamine agonists are another class of medications that are widely used to treat Parkinson's. These medications look and act like dopamine in the brain, so are essentially an artificial dopamine substitute, but are not exactly dopamine. Examples of the dopamine agonists include NeuproTM (rotigotine), RequipTM (ropinirole), and MirapexTM (pramipexole). These medications are clearly effective, and for more advanced cased of Parkinson's often form part of the treatment regimen. They have a much higher rate of causing hallucinations than carbidopa/levodopa, and are also associated with an "impulse control" disorder, which affects around 9% of people taking them. This impulse control disorder manifests as such things as excessive gambling, excessive online shopping, or watching X rated movies. For these reasons, at Bridgepoint Neurology we virtually always start treatment for Parkinson's with some form of carbidopa/levodopa and slowly increase the dose to ensure tolerability. We usually switch to a dopamine agonist if there is an intolerable side effect to carbidopa/levodopa. Additionally, we may add one of these medications to an existing regimen of carbidopa/levodopa to improve symptom control.
Some patients with Parkinson's are prescribed amantadine, which now has a couple branded long acting forms, GocovriTM and Osmolex ERTM. This medication is modestly helpful for the motor manifestations, but also has a high frequency of hallucinations. However, it can be helpful at reducing dyskinesias, a form of fluctuation that is discussed in the section on fluctuations below.
AzilectTM is another medication approved for Parkinson's. It works by inhibiting one of the breakdown pathways of dopamine in the brain. Its beneficial effects are modest, but for some people it provides just the right amount of fine tuning without having increasing side effects from higher doses of carbidopa/levodopa or the dopamine agonists. It may also prolong the duration of effectiveness of each dose of carbidopa/levodopa a little bit.
Some patients with Parkinson's are still prescribed ArtaneTM (trihexyphenidyl), and this is an outdated and minimally effective medication for Parkinson's.