A contraindication is a specific situation in which a drug, procedure, or surgery should not be used because it may be harmful to the person.
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.
There are two types of contraindications:
Relative contraindication means that caution should be used when two drugs or procedures are used together. (It is acceptable to do so if the benefits outweigh the risk.)
Absolute contraindication means that event or substance could cause a life-threatening situation. A procedure or medicine that falls under this category should be avoided.