Injection compatibility: Introduction

Last updated: Monday, July 13, 2015

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An incompatibility occurs after mixing parenteral drugs if at least one of them becomes less effective (or more toxic, although this is rare). Changes that can occur include:

Physical incompatibility (e.g. precipitation, haziness, crystallisation, emulsions ‘cracking’). This is largely determined by the pH and formulation of the injections concerned.

Chemical incompatibility (e.g. degradation, inactivation, or a new compound formed). This is a chemical reaction between the drugs after mixing.

   Precipitation - an example of physical incompatibility 
Courtesy of Simon Wills
This neonatal TPN was formulated incorrectly and calcium has crystallized out in the far right  port 
Courtesy of Peter Rhodes

Note that many published studies of compatibility only examine physical incompatibility – i.e. there is a visual inspection of the mixture only, without chemical analysis. Chemical incompatibility must be determined by assay of the mixture (e.g. by HPLC). Physical incompatibility, particularly precipitation, is also the only incompatibility that might be easily detected at ward level. So, just because a mixture ‘looks okay’ does not necessarily mean that the drugs are compatible!