I just read that it is the vomiting center located in the brainstem which is responsible for nausea and vomiting while taking levodopa. It contains nerve cells whose job is to detect toxin in the circulation and provoke stomach to vomit to prevent further absorption, and it is sensitive to levodopa and many other substances. If a person is absorbing levodopa especially well even small amounts can provoke this response. In the beginning taking small amounts of levodopa only after meals to slow its absorption might lessen this response with time (could be 3 to 6 months).