In medicine, a contraindication is a condition or factor that serves as a reason to withhold a certain medical treatment. Some contraindications are absolute, meaning that there are no reasonable circumstances for undertaking a course of action. For example, a person with an anaphylactic food allergy should never eat the food to which they are allergic. Similarly, a person with hemochromatosis should not be administered iron preparations.
Other contraindications are relative, meaning that the patient is at higher risk of complications, but that these risks may be outweighed by other considerations or mitigated by other measures. For example, a pregnant woman should normally avoid getting X-rays, but the risk may be outweighed by the benefit of diagnosing (and then treating) a serious condition such as tuberculosis.wikipedia definition