When a drug loses its effectiveness in a patient, what is this condition called?

The condition where a drug loses its effectiveness in a patient is referred to as tachyphylaxis. This phenomenon occurs when the body's response to a drug diminishes rapidly after administration, even if the drug is continued at the same dose. Tachyphylaxis can happen due to various mechanisms, such as receptor desensitization, depletion of mediators, or alterations in drug metabolism, leading to a reduced pharmacological effect over time.

Tolerance, while related, refers to the gradual decrease in the drug's effectiveness that develops after prolonged use, rather than a rapid decrease. An allergic reaction is an immune response to a drug characterized by symptoms like rash or anaphylaxis, and idiosyncrasy refers to an abnormal reaction to a drug that is peculiar to an individual and does not fit within expected pharmacological responses.