offer a clear classification for these concepts. It is possible that authors would misapply any given classification. Direct interactions between drugs are also possible and may occur when two drugs are mixed before intravenous injection. For example, mixing Thiopentone and Suxamethonium can lead to the precipitation of thiopentone. The change in an organism's response upon administration of a drug is an important factor in pharmacodynamic interactions. These changes are extraordinarily difficult to classify given the wide variety of modes of action that exist, and the fact that many drugs can cause their effect through several different mechanisms. This wide diversity also means that in all but the most obvious cases, it is important to investigate and understand these mechanisms. A well-founded suspicion exists that there are more unknown interactions than known ones. Pharmacodynamic interactions can occur on: Pharmacological receptors: Receptor interactions are the most easily defined, but they are also the most common. From a pharmacodynamic perspective, two drugs can be considered to be: Homodynamic, if they act on the same receptor. They, in turn, can be: Pure agonists, if they bind to the main locus of the receptor, causing a similar effect to that of the main drug. Partial agonists if, on binding to one of the receptor's secondary sites, they have the same effect as the main drug, but with a lower intensity. Antagonists, if they bind directly to the receptor's main locus but their effect is opposite to that of the main drug. These include: Competitive antagonists, if they compete with the main drug to bind with the receptor. The amount of antagonist or main drug that binds with the receptor will depend on the concentrations of each one in the plasma. Uncompetitive antagonists, when the antagonist binds to the receptor irreversibly and is not released until the receptor is saturated. In principle, the quantity of antagonist and agonist that binds to the receptor will depend on their concentrations. However, the presence of the antagonist will cause the main drug to be released from the receptor regardless of the main drug's concentration, therefore all the receptors will eventually become occupied by the antagonist. Heterodynamic competitors, if they act on distinct receptors. Signal transduction mechanisms: these are molecular processes that commence after the interaction of the drug with the receptor. For example, it is known that hypoglycaemia (low blood glucose) in an organism produces a release of catecholamines,