What Happens If Antivenom Is Administered When It Is Not Necessary? Myths, Risks, and Reality

fundacionvivarium
Jan 9
4 min read

In the public WhatsApp group of Fundación Vivarium, one of our members raised an excellent and very clinically relevant question:

“If antivenom were administered to a patient who does not actually require it — for example, after a bite from a non-venomous snake, a dry bite, or in the absence of proper diagnostic tests — would that antivenom simply represent an unnecessary loss, or could it cause adverse reactions in the patient solely because it was not needed?”

In addition to answering the group member, we decided to expand the explanation on our blog in order to address the topic in greater detail.

What is antivenom and why can it cause reactions?

Antivenom is a biological product that contains immunoglobulins (antibodies) derived from animals (horses, in the case of Venezuela) that have been immunized with snake venom. These proteins work by neutralizing the venom in patients who have truly been envenomed and who present clear clinical symptoms.

Because these proteins are foreign to the human body, the immune system may react against them, regardless of whether the antivenom was actually needed or not. In other words, the risk of a reaction does not appear or disappear simply because the patient was or was not envenomed. It depends on the antivenom itself and on how the body responds to these heterologous proteins.

What types of reactions can occur?

Reactions to antivenoms are generally classified into two main categories:

1. Early (immediate) reactions

These may occur during the infusion or within the first 24 hours. They include:

Urticaria, itching, flushing
Nausea, vomiting
Difficulty breathing or bronchospasm
Hypotension
In severe cases: anaphylaxis, which can be life-threatening if not treated promptly

These reactions occur because the immune system recognizes the antivenom proteins as foreign and triggers an allergic or hypersensitivity response.

2. Late reactions (serum sickness)

These occur several days after administration (between 5 and 14 days or more) and may present as:

Fever
Skin rashes or urticaria
Joint and muscle pain
Swollen lymph nodes
Flu-like symptoms

These are caused by the formation of immune complexes between the antibodies contained in the antivenom and the antibodies produced by the patient against them.

What does this mean for someone who was not truly envenomed?

If antivenom is administered to a patient who was not envenomed:

The clinical benefit is null, because there was no venom to neutralize.
The antivenom does not “activate” simply by being present in the body: its therapeutic usefulness depends on the presence of venom. Antivenom is not prophylactic and should not be administered “just in case.”
However, adverse reactions may still occur, because the body recognizes the foreign proteins contained in the antivenom.

Therefore, it is incorrect to assume that if the antivenom “was not useful,” then it also carries no risk: it does carry risk.

Why should antivenom only be administered in controlled settings?

For all these reasons:

Antivenom should only be given after careful clinical evaluation, based on signs, symptoms, evidence of envenomation, or laboratory findings.
It must be administered in a hospital setting with medical supervision, by personnel and with equipment prepared to manage possible adverse reactions (epinephrine, antihistamines, corticosteroids, and resuscitation measures).

Can adverse reactions to antivenom be reduced?

Yes, they can. Advances in antivenom production have made it possible to significantly reduce adverse reactions without affecting therapeutic efficacy. Older antivenoms used whole immunoglobulins (IgG), which more easily activated the complement system and increased the risk of allergic reactions.

Modern antivenoms now use smaller antibody fragments such as F(ab’)₂ or Fab, which have a better safety profile and a lower frequency of adverse events.

In Latin America, countries such as Costa Rica, Brazil, and Colombia produce highly purified antivenoms with strong clinical evidence of safety and efficacy.

Mexico stands out for the development of state-of-the-art fabotherapeutics that are highly specific. In addition, alternatives such as chicken-derived IgY antibodies are being studied, which could further reduce allergic reactions.

In conclusion, adverse reactions are not inevitable. They can be minimized through modern antivenoms, proper clinical indication, and appropriate administration. The goal is not to avoid antivenom, but to continue improving its safety.

Conclusion

Administering antivenom when it is not necessary provides no therapeutic benefit and may, in fact, expose the patient to risks inherent to this biological product, such as immediate allergic reactions or serum sickness days later. For this reason, its use is reserved for cases in which there is clear clinical evidence of envenomation and it is properly indicated by trained medical personnel.

¿Quieres seguir debatiendo estos temas con profesionales y entusiastas de la fauna silvestre y la medicina?👉 Únete a nuestro grupo público de WhatsApp de Identificación y Avistamiento de Serpientes de Venezuela: bit.ly/45LU8C5 Allí respondemos preguntas reales y compartimos experiencias con base científica.

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