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Calan (verapamil) is a medication that can lower both the heart rate and the blood pressure.  The adverse effects that you are describing may be caused be a problem with the heart rate or rhythm, such as sick sinus syndrome.  The other possibility is that your grandmother may not be able to tolerate the medication anymore due to a problem with maintaining her blood pressure, which can be caused by a dysautonomia or dysfunction of the autonomic nervous system, an infection, or different drug interaction.  Speak with her doctor to try to get to the bottom of it.