A contraindication is a specific situation in which a drug, procedure, or surgery should not be used because it may be harmful to the patient.
There are two types of contraindications:
Some treatments may cause unwanted or dangerous reactions in people with allergies, high blood pressure, or pregnancy. For example, isotretinoin, a drug used to treat acne is absolutely contraindicated in pregnancy due to the risk of birth defects. Certain decongestants are contraindicated in people with high blood pressure and should be avoided.
Many medications should not be used together by the same person. For instance, a person who takes warfarin to thin the blood should not take aspirin, which is a blood thinner. This is an example of a relative contraindication.