Sequential randomised and double blind trial of promethazine prophylaxis against early anaphylactic reactions to antivenom for bothrops snake bites

BMJ 1999;318:1451-1452 ( 29 May )

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Sequential randomised and double blind trial of promethazine prophylaxis against early anaphylactic reactions to antivenom for bothrops snake bites

Hui Wen Fan, doctor, ^a Luiz F Marcopito, associate professor, ^b João Luiz C Cardoso, doctor, ^a Francisco O S França, doctor, ^a Ceila M S Malaque, doctor, ^a Ronnei A Ferrari, doctor, ^a Robert David G Theakston, associate professor, ^c David A Warrell, associate professor. ^d

^a Hospital Vital Brazil, Instituto Butantan, Avenue Vital Brazil 1500, 05503-900, São Paulo, Brazil, ^b Division of Epidemiology, Escola Paulista de Medicina, Unifesp, 04039-032, São Paulo, Brazil, ^c Alistair Reid Venom Research Unit, Liverpool School of Tropical Medicine, Pembroke Place, Liverpool L3 5QA, ^d Centre for Tropical Medicine, University of Oxford, John Radcliffe Hospital, Headington, Oxford OX3 9DU

Correspondence to: Dr H W Fan fhui{at}uol.com.br

Objective: To investigate the efficacy of the H₁antihistaminepromethazine against early anaphylactic reactions toantivenom.
Design: Sequential randomised, double blind, placebocontrolledtrial.
Setting: Public hospital in a venom research institute,São Paulo,Brazil.
Participants: 101 patients requiring antivenom treatmentafter being bitten by bothropssnakes.
Intervention: Intramuscular injection of promethazine (25mg for adults and 0.5/kg for children) or placebo given 15-20min before starting intravenous infusion ofantivenom.
Main outcome measures: Incidence and severity of anaphylactic reactionsoccurring within 24 hours afterantivenom.
Results: Reactions occurred in 12 of 49 patients treatedwith promethazine (24%) and in 13 of 52 given placebo (25%); mostwere mild or moderate. Continuous sequential analysis indicatedthat the study could be interrupted at the 22nduntied pair, without preference for promethazine orplacebo.
Conclusion: Prophylaxis with promethazine does not preventearly reactions. Patients should be observed carefully duringantivenom infusion and the subsequent fewhours.

Key messages

Antivenom therapy may cause early anaphylactic reactions
Various drugs are used to prevent reactions, but none have been tested in randomised controlled studies
This study showed that promethazine is not better than placebo at preventing early reactions
Although most reactions are mild or moderate, trials of other drugs should be done to reduce frequency of anaphylaxis

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