A Jarisch–Herxheimer reaction (English: /ˌjɑːrɪʃ ˈhɛərkshaɪmər/) is a reaction to endotoxin-like products released by the death of harmful microorganisms within the body during antibiotic treatment. Efficacious antimicrobial therapy results in lysis (destruction) of bacterial cell membranes, and in the consequent release into the bloodstream of bacterial toxins, resulting in a systemic inflammatory response.
Jarisch–Herxheimer reactions are usually not life-threatening.

The Jarisch–Herxheimer reaction is classically associated with penicillin treatment of syphilis. Duration in syphilis is normally only a few hours. The reaction is also seen in other diseases caused by spirochetes, such as borreliosis (Lyme disease and tick-borne relapsing fever) and leptospirosis, and in Q fever.[1] Similar reactions have also been reported to occur in bartonellosis (including cat scratch disease),[2][3] brucellosis,[4] typhoid fever,[5] trichinosis,[6] and cerebral trypanosomiasis.[7]