Citrus juices, specially grapefruit,
CRANBERRY and orange juice, interact with heart
and blood pressure medications.
By inhibiting the same enzyme that metabolizes the Metroprolol, the juice raises the levels of the drug in your blood and it can cause an overdose.
Your doctor should have known that.
Page 6 talks about the interaction of medications with cranberry juice......
http://www.fda.gov/cder/drug/drugInteractions/pubs/2004_Huang_Lesko_drug-food_interactions.