A 49-year-old man after partial knee replacement who has Parkinson's Disease and is taking co-careldopa; post-op meds include dalteparin, co-codamol, and metoclopramide. What is causing his current symptoms?

• A. Increased effect due to pharmacodynamic synergy
• B. No interaction
• C. Decreased effect due to pharmacodynamic interaction
• D. Metoclopramide increases Levodopa absorption

Answer: (C)

Dopamine receptor blockade by metoclopramide reduces the effect of levodopa in Parkinson’s disease. Levodopa provides brain dopamine to control symptoms, but metoclopramide blocks central dopamine D2 receptors, counteracting levodopa’s action and leading to a diminished therapeutic effect (worsening motor symptoms). This is a pharmacodynamic interaction—drugs altering each other’s effects at the site of action—rather than a change in drug levels or absorption. The idea that metoclopramide increases levodopa absorption isn’t the primary mechanism here, and assuming no interaction or a synergistic increase would not explain the observed symptoms.