Sinemet (Carbidopa-Levodopa) This combination medication is used to treat symptoms of Parkinson's disease or Parkinson-like symptoms (such as shakiness, stiffness, difficulty moving). Parkinson's disease is thought to be caused by too little of a naturally occurring substance (dopamine) in the brain. Levodopa changes into dopamine in the brain, helping to control movement. Carbidopa prevents the breakdown of levodopa in the bloodstream so more levodopa can enter the brain. Carbidopa can also reduce some of levodopa's side effects such as nausea and vomiting.