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 agonism. If one drug activates the receptor and the other drug inhibits the receptor, this is called antagonism.
Synergism is the situation in which two different drugs both activate the same physiological process by different mechanisms (and therefore could theoretically activate the system to a greater extent than either could alone).
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.