adverse reaction ad·verse reaction (ād-vûrs', ād'vûrs')
A result of drug therapy that is neither intended nor expected in normal therapeutic use and that causes significant, sometimes life-threatening conditions.
The FDA shuts down trials all the time because people have an adverse reaction or die.
Reeve passed away after experiencing an adverse reaction to an antibiotic on Oct. 10, 2004.
The causal mechanism of this adverse reaction to increased vessel traffic remains unknown.