A contraindication (pronounced as contra-indication) is a condition or factor that speaks against a certain measure. It is mostly used in medicine, with regard to factors that increase the risks involved in using a particular drug, carrying out a medical procedure, or engaging in a particular activity.

Some contraindications are absolute, meaning that there are no reasonable circumstances for undertaking a course of action. For example, a baby with a fever should never be given aspirin because of the risk of Reye's syndrome, and 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 far less than the risk of not diagnosing or being able to treat a serious condition such as tuberculosis or a broken bone. Relative contraindications may also be referred to as cautions, such as in the British National Formulary.