I take diltiazem and Diovan HCT for blood pressure. Is it ok to take Benadryl for hives?
According to the drug interaction checker, Benadryl can increase the effects of diltiazem, which could cause your blood pressure to drop too much. It advises close monitoring of blood pressure if they are taken together. Ideally, I would say to check with your doctor before taking the Benadryl. Since it's after hours, perhaps you could call your local Ask a Nurse or a pharmacist at an all-night pharmacy for advice.
Medication Overload: Watch for Drug Interactions
Understanding Low Blood Pressure
What Your Blood Pressure Should Be