An interesting population-level signal is coming from Barcelona.
Antihistamines, Long COVID and thrombosis🧵
Antihistamines, Long COVID and thrombosis🧵
https://twitter.com/ahandvanish/status/2003199122497781919
A large real-world preprint analysis (~193,000 people, 2020–2025) suggests that people chronically taking antihistamines had
fewer thrombotic events
and almost no recorded long COVID - even after multiple reinfections.
This is not proof. But it is not noise either.
fewer thrombotic events
and almost no recorded long COVID - even after multiple reinfections.
This is not proof. But it is not noise either.
First, the basics.
This is an observational study, not a randomized trial.
So it cannot establish causality.
What it can do is identify patterns worth taking seriously - especially when they are consistent across subgroups.
This is an observational study, not a randomized trial.
So it cannot establish causality.
What it can do is identify patterns worth taking seriously - especially when they are consistent across subgroups.
What stood out most strongly?
Thrombosis
Among people with at least one chronic medication
antihistamine users ~1.6%
non-users ~3.3%
Thrombosis
Among people with at least one chronic medication
antihistamine users ~1.6%
non-users ~3.3%
The difference was statistically robust and persisted across age, sex, vaccination status, and infection history.
This is the strongest signal in the dataset.
This is the strongest signal in the dataset.
Long COVID
In people not using antihistamines, long COVID prevalence increased with each reinfection - especially after the third.
In people not using antihistamines, long COVID prevalence increased with each reinfection - especially after the third.
In the antihistamine group
no recorded long COVID cases after 2 or ≥3 infections
Numbers are smaller here, so caution is required - but the pattern is striking.
no recorded long COVID cases after 2 or ≥3 infections
Numbers are smaller here, so caution is required - but the pattern is striking.
Naturally, the next question is
Do we have RCTs for this?
Short answer. No - not for prevention.
Do we have RCTs for this?
Short answer. No - not for prevention.
So how should this be interpreted?
Not as a miracle cure.
But as a biologically plausible signal -
histamine pathways affect neuroinflammation
antihistamines can modulate platelet-activating factor (PAF)
PAF is linked to microthrombosis and vascular inflammation
This fits existing biology.
Not as a miracle cure.
But as a biologically plausible signal -
histamine pathways affect neuroinflammation
antihistamines can modulate platelet-activating factor (PAF)
PAF is linked to microthrombosis and vascular inflammation
This fits existing biology.
The uncomfortable reality - antihistamines are cheap, generic, off-patent.
Which means large preventive RCTs are unlikely without public funding.
Absence of RCT evidence here reflects lack of incentives, not lack of relevance.
Which means large preventive RCTs are unlikely without public funding.
Absence of RCT evidence here reflects lack of incentives, not lack of relevance.
This preprint does not prove anything -
but it raises a clear, coherent, and testable hypothesis.
Ignoring such signals is not scientific caution.
It is scientific inertia.
but it raises a clear, coherent, and testable hypothesis.
Ignoring such signals is not scientific caution.
It is scientific inertia.
PuigdellĂvol-Sánchez et al., Long COVID and Reduced Thrombosis in Antihistamine Treated Patients.
Preprints.org, December 2025. preprints.org/manuscript/202…
Preprints.org, December 2025. preprints.org/manuscript/202…
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