In this episode, I’ll discuss why IV phenytoin cannot be diluted lower than 5 mg/mL.
When it comes time to convincing a clinician to do or not do something, the “why” often becomes very important. This is for good reason, as there are many examples of unnecessary information placed into prescribing information for medications.
The prescribing information for phenytoin includes instructions to not dilute the IV solution below 5 mg/mL when using normal saline as the diluent. This is somewhat counter-intuitive because for the most part, high dilutions of medications are more often problematic, while lower dilutions are often less problematic.
IV phenytoin is different because the volume of the diluent plays a role in the chance of crystallization of IV phenytoin when mixed in normal saline. As the concentration of phenytoin drops below 5 mg/mL, the pH of the solution
exceedsfalls below 10, and the chance of crystallization increases. As discussed in episode 458, particulates from drug incompatibilities are receiving more attention as a possible cause of morbidity in critically ill patients.
Shout out to “Pharmacy Sadaf” for inspiring this episode and “Pharmacy Elizabeth” for pointing out that crystalization happens as pH falls below 10, not exceeds.
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