Drug pharmacodynamic interactions
Pharmacodynamic interactions can occur anywhere from the physiological level all the way down to the level of the ligand-receptor binding. Two different drugs can compete for the same receptor. If both drugs are agonists, this is competitive
An example of antagonism is the addition of the antipsychotic agent haloperidol (D2-antagonist) to a Parkinson’s patient on levodopa (prodrug of dopamine). The likely result is:
Extra info: The addition of haloperidol will block all the beneficial effects of levodopa in the Parkinson’s patient. And thus the original symptoms of Parkinson’s disease (rigidity) will appear again.