Injection compatibility: Reducing risk

Last updated: Thursday, November 18, 2021

Injectable medicines can be mixed in a variety of ways:

  • In the same bag of intravenous fluid (sometimes called 'admixture').
  • In the same syringe, diluted, for a subcutaneous infusion.
  • In the same syringe, neat, for intramuscular injection.
  • In the same intravenous line, or cannula.

Avoiding the need for mixing

The easiest way to solve an incompatibility problem is to think about ways to avoid mixing. You can remember these with the acronym: NATO ( Necessary? Alternative route? Timing? Other drugs?):


Can some non-essential drugs be stopped? Is there any unnecessary duplication (e.g. you should not need to mix co-amoxiclav and metronidazole because their antibacterial spectrums overlap).

Is the oral route an option?
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Alternative routes?

Can an alternative route be used to avoid IV administration? For example:

  • Insulin may be given subcutaneously instead of IV.
  • Metronidazole can be given rectally instead of IV.
  • Cefuroxime can be given by IM injection instead of IV.
  • Fluconazole has excellent bioavailability orally compared to IV.
  • If the patient has a nasogastric tube, many drugs are available as oral syrups or solutions.
  • Many drugs can be given by IV injection as well as by infusion. Since IV injections are quicker, this may avoid the need for mixing.


Changing the timing of drug administration may avoid the need for mixing.

  • Can short IV infusions be given sequentially (i.e. one after the other) to avoid mixing? This can often be done with antibiotics. Remember to flush between each drug.
  • Can a less urgent continuous infusion (e.g. pantoprazole) be interrupted temporarily to allow administration of a short infusion of a second drug?

Can you choose a different medicine?
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Other drugs?

Can a different drug be used to overcome a compatibility problem? For example:

  • An IV injection of teicoplanin instead of a long IV infusion of vancomycin.
  • Subcutaneous tinzaparin can be given instead of an IV infusion of heparin to treat PE.
  • Sublingual lorazepam instead of IV midazolam or diazepam for anxiety.
  • Sublingual buprenorphine instead of a parenteral opioid for pain.
  • Buccal prochlorperazine instead of a parenteral antiemetic.

Inserting another intravenous line 

If these strategies are not practical or clinically inappropriate, then it may be necessary to attempt to gain additional intravenous access by inserting another line into the patient in order to administer potentially incompatible medicines. However this is not without risk or discomfort for the patient, and some individuals may be more difficult to cannulate (e.g. children, those who have undergone previous multiple cannulations, patients who are dehydrated).

Quick ward question 
A nurse tells you that IV infusions of amiodarone and heparin running into the same line have ‘gone all white’; what should she do?    Consider, then click for answer.
Advise her to stop the infusion immediately and inform the doctors caring for the patient straight away. If the IV infusion line has a filter in it then the precipitate won’t have reached the patient’s bloodstream, but they won’t have received some or all of the medicines prescribed. You can then help the team to come up with an alternative plan quickly since these are both essential medicines (e.g. insert a separate line? Subcutaneous LMWH? Oral amiodarone?)