MCAS Antihistamine Notes
Oct. 6th, 2026 01:50 pmThis is not medical advice.
It is a summary of
synecdochic's bluesky thread, which is also not medical advice. The bluesky thread contains additional instructions for people taking different antihistamines, people with kidney problems or QT prolongation, and people taking their first dose of vaccine. This summary is primarily for people who want to go up to the maximum dose of cetirizine (IE: me, it's for me, I want a copy for myself to refer back to easily).
- Obtain cetirizine 10mg and famotidine 20mg, available over the counter in the US.
- Set reminders, ideally 12 hours apart, one for morning and one for night.
Vaccination
- 2-3 hours before appointment: 2 cetirizine
Active infection
- Morning: 4 cetirizine, 1 famotidine
- Night: 4 cetirizine, 1 famotidine
Continue while symptoms of COVID persist.
Daily
Maximum daily dose:
- Morning: 4 cetirizine (4x10=40mg), 1 famotidine (1x20mg)
- Night: 4 cetirizine (4x10=40mg), 1 famotidine (1x20mg)
Lower sustainable dose for rationing:
- Morning: 2 cetirizine (2x10=20mg), 1 famotidine (1x20mg)
- Night: 2 cetirizine (2x10=20mg), 1 famotidine (1x20mg)
If further rationing is necessary, drop the famotidine first.
Taper up morning cetirizine as slow as one pill every two weeks to manage drowsiness.