Intravenous fluids offer special scope for interactions (incompatibilities) when drugs are added to the reservoir or syringe, for a number of reasons. Drugs commonly are weak organic acids or bases. They are often insoluble and to make them soluble it is necessary to prepare salts. Plainly, the mixing of solutions of salts can result in instability which may or may not be evident from visible change in the solution, i.e. precipitation. Furthermore, the solutions have little buffering capacity and pH readily changes with added drugs. Dilution of a drug in the reservoir fluid may also lead to loss of stability.
A serious loss of potency can result from incompatibility between an infusion fluid and a drug that is added to it. Issues of compatibility are complex but specific sources of information are available in manufacturers' package inserts, formularies or from the hospital pharmacy (where the addition ought to be made). The general rule must be to consult these sources before ever adding a drug to an infusion fluid or mixing in a syringe.
Mixing drugs formulated for injection in a syringe may cause interaction, e.g. protamine zinc insulin contains excess of protamine which binds with added soluble insulin and reduces the immediate effect of the dose.
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