Three kinds of interactions between drugs can occur: Pharmaceutical interactions take place between drugs before administration. For example, a precipitate can be formed when two drugs are mixed in a solution (i.e. phosphates and calcium). Pharmacokinetic interactions take place when one drug alters
the kinetics of another drug (ciprofloxacin inhibits the metabolism of coumarins). Pharmacodynamic interactions usually take place at the level of receptors and are thus molecular interactions in a physiological perspective (propranolol inhibits the effectiveness of salbutamol in asthma).
Extra info: Enalapril and triamterene can both increase serum potassium concentrations leading to the significant adverse effect of cardiac arrhythmias. Since this is an alteration in physiologic response, this is a pharmacodynamic interaction.