To stabilize our very overactive mast cells, we need the antihsitamines twice a day. Our mast cells are too reactive to just take the medicine once a day.

The masto specialists have found that a combination of different antihistamines work better than the same one twice a day.

Singular, by the way, is not an antihistamine. It is an anti-leukotriene and seems to help chase away many of the remaining symptoms that the antihistamines don't clear up.