A small glass of 100% fruit juice is a good part of a healthful breakfast, right? If you're on certain prescription medications, your fruit juice may be doing more harm than good.

Grapefruit juice, for example, can interfere with blood pressure medication and actually increase its efficacy, subsequently creating a risk of overdose. Conversely, recent research suggests that grapefruit juice may prevent certain anti-allergy medications from absorbing into the bloodstream properly. A component of grapefruit juice called naringin appears to be the culprit.

Researchers state that this finding may be the "tip of the iceberg" as other juices, including orange and apple, also contain naringin-like substances that may have similar effects.