Positives

• A patient who had been taking Allegra (fexofenadine) for seasonal allergies noticed marked improvement in brain fog and fatigue when she added Benadryl, eventually combining both to regain about 95% of her pre-COVID functioning PMC.
  
  

Negatives

• A meaningful subset of Long COVID patients try multiple antihistamines (fexofenadine, cetirizine, loratadine) for weeks without noticing any improvement in their symptoms, often concluding mast cell symptoms aren’t the right target for their case Frontiers in Cardiovascular Medicine.

Hurdles & Side Effects

• As with other antihistamines for MCAS-overlap Long COVID, dosing typically needs to go above standard 180 mg/day to be effective, which is off-label and may require an allergist or immunologist to prescribe RTHM.