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.