Can fruit juices block certain medicines from being effective?

It’s always best to check labels, and, when taking medications, to check with the doctor about drug interactions.  Some fruit juices will prevent certain medications from reaching the bloodstream or reduce absorption of the medication.  For example, research has found that orange and apple juices can reduce absorption of an anti-cancer agent and certain drugs used to prevent the rejection of transplant organs.
 
Source(s): 
Complete Food and Nutrition Guide by Roberta Lawson Duyff, 2006.