What is the effect of the addition of carbidopa to levodopa? (Select all that apply. One, some, or all)

A. Carbidopa inhibits peripheral metabolism of levodopa, allowing more levodopa to reach the brain without increasing peripheral side effects. This enables clinicians to increase levodopa doses if needed for therapeutic benefit while minimizing systemic adverse effects. B. While carbidopa reduces peripheral conversion of levodopa (which decreases nausea, vomiting, and hypotension), it does not prevent central adverse effects such as dyskinesias (abnormal movements) or psychiatric symptoms like hallucinations. These effects occur in the CNS where carbidopa has little impact. C. Carbidopa blocks the enzyme dopa-decarboxylase in the periphery, preventing levodopa from converting to dopamine in the intestines and other tissues outside the CNS. This increases the amount of levodopa available to cross the blood-brain barrier. D. By reducing peripheral dopamine formation, carbidopa decreases adverse effects such as nausea and vomiting, which are common when dopamine acts on the chemoreceptor trigger zone in the gut and medulla. E. Carbidopa does not cross the blood-brain barrier effectively. Its role is peripheral enzyme inhibition, not direct therapeutic action in the CNS. The therapeutic effect comes from levodopa conversion to dopamine in the brain.