A new and very interesting EMR "glitch" - no warnings on stopping a medicine that diminishes the effects of a second medication

A new and very interesting EMR "glitch" from a report I received recently:

... I found a glitch with my [name redacted] EMR. It probably happens with all EMRs. I had a patient on primidone (http://en.wikipedia.org/wiki/Primidone) for essential tremor. Later, his primary care put her on warfarin [a "blood thinner" - ed.] for atrial fibrillation. Some time after that, I took her off of primidone.  Her INR jumped to 7 or 8. [High - ed.] What happens is that the EMRs warn a physician pretty well if you START a medicine that interacts with warfarin, but fails to warn if you STOP a medicine that interacts with warfarin. If you are used to relying on your EMR to warn you about drug interactions, you can fall into this trap easily, as I found out. Luckily, the patient was not harmed.

In other words, if a medication that interacts with another medication by suppressing the latter's effects to some degree is discontinued, EMRs may not warn of it.  Stopping the former can accentuate the effects of the latter, and disaster can result.  A primidone metabolite, phenobarbital, decreases INR and the anticoagulant effects of warfarin (http://www.medscape.com/viewarticle/745645_3).  Stop primidone, but continue warfarin, and ... wham.

The alert algorithms were apparently not designed with this eventuality in mind ... probably because the designers never thought of this issue.  Medicine is not as easy as it might appear to the outside, non-expert observer.

-- SS