Parkinson’s disease can be very debilitating and not understanding how medications should be dosed can compound the problem. Let’s review the pharmacology briefly and why you need to be careful with Sinemet 10/100.
In Parkinson’s, we have a dopamine shortage. Levodopa is converted to dopamine in the brain but before it can get to the central nervous system, it must pass from systemic circulation across the blood-brain barrier. If carbidopa is not coadministered with levodopa, levodopa is converted to dopamine in the periphery before it can get to the brain. Carbidopa prevents the peripheral breakdown of levodopa by blocking DOPA-decarboxylase.
With that bit of physiology/pharmacology taken care of, what does that mean clinically? If we dose levodopa without adequate carbidopa, two major negative outcomes can happen.
- More levodopa gets converted to dopamine in the periphery and can lead to more side effects like nausea.
- Inadequate amounts of levodopa get to the brain because it is broken down by peripheral DOPA-decarboxylase.
This leads to the next question. How much carbidopa is necessary?
Without coadministration of carbidopa, the % of levodopa that reaches the CNS is estimated at less than 1%. In studies, it has been shown that approximately 70-100 mg/day of carbidopa is necessary to effectively inhibit the activity of peripheral dopa-decarboxylase. This is why the typical starting dose is carbidopa 25 mg and levodopa 100 mg three times daily. Note that if we “go easy” with the carbidopa and use Sinemet 10/100 dosing, we may actually experience more side effects (in the form of nausea) and also be unlikely to have efficacy in treating Parkinson’s symptoms.
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