In medicine, a contraindication is a condition or factor that increases the risk involved in using a particular drug, carrying out a medical procedure or engaging in a particular activity.

• An absolute contraindication is a condition that prohibits the use of a treatment altogether. For example, an untreated pneumothorax would be an absolute contraindication to hyperbaric oxygen treatment.
• A relative contraindication weighs in against the use of a treatment when assessing its risk/benefit ratio. For example, a history of peptic ulcer is a contraindication towards taking aspirin. If, however, the benefit of using aspirin is seen as greater than the risk of a recurrence of the ulcer, and no reasonable alternatives are available, the treatment is still indicated.