I've heard that some foods can make medications less effective, is that true?

Drug-food interactions are possible. For example, antibiotics like tetracycline should not be taken with diary products. The calcium in these foods reduces the absorption of the antibiotic, making it less effective. Some drugs work best when taken with food, while others should be taken on an empty stomach. Read labels carefully and ask your health care provider or pharmacist about possible interactions with food.

Gail Carlson, MPH Ph.D., Continuing Medical Education, School of Medicine, 
University of Missouri-Columbia