Injection compatibility: Predicting incompatibility

Last updated: Thursday, November 18, 2021

Often there are little published data available to help with compatibility decisions. If there is inadequate information you must always inform the enquirer of this. When no information at all is available, mixing should be avoided. However, sometimes it is important to at least offer some guidance to ward staff if a certain number of drugs will have to be mixed despite a lack of knowledge of their compatibility.

The questions below may help if the points in the Reducing risk section do not help you to avoid mixing injectable medicines:

  • Do the drugs have a similar pH? Drug solutions with very different pH values are less likely to be compatible than those with similar pH values. There is a table of common injections showing their pH here.
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  • Are other chemically similar drugs compatible? For example, if a study showed that morphine was compatible with ondansetron, you would have more confidence that diamorphine might be compatible with ondansetron.

  • Do the injections have a similar formulation? Drugs with very different formulations are unlikely to be compatible. For example, oil-based injections such as Depixol are unlikely to be compatible with aqueous injections.

  • Are individual drugs or formulations intrinsically stable? Drugs that are naturally very unstable, or which require very precise formulation to ensure stability, may be more likely to be incompatible with other drugs (e.g. Diazemuls).

  • Are any of the injections new products? There are often very little data on compatibilities for drugs that are completely new chemical entities or very novel formulations. You should be extremely careful about mixing these drugs with anything.

  • What concentrations are being used? Most information sources give compatibility data based on specific concentrations of the drugs involved. If the concentrations you encounter are different from these, you need to interpret the published data with caution. The stability of some drugs varies significantly depending upon concentration (e.g. co-trimoxazole, phenytoin).

  • What diluents are being used? Sometimes incompatibilities arise because a drug is not compatible with the diluent used for the second drug. If the diluent can be changed, then the problem might be avoided. For example amiodarone is incompatible with sodium chloride 0.9%.

Quick ward question 
A nurse asks you if intravenous soluble insulin is compatible with intravenous furosemide? The patient has a triple lumen central line and the infusions will be running through separate lumens of the line    Consider, then click for answer.
The lumens of a triple lumen line are effectively separate catheters bound together. So medicines that enter via one lumen do not mix in the catheter with medicines administered via one of the other lumens. The lumens exit the catheter at different points along its length and any medicines administered down them are rapidly diluted by the fast-flowing bloodstream. Therefore you can advise the nurse that there is no need to check on compatibility between insulin and furosemide as the drugs will not mix in the line. See our page on intravenous access devices for more information.