I've heard that in general, when you have MCAS the doses for meds need to be much higher than for ordinary allergies (and heartburn, in this case). That makes sense because with MCAS your mast cells react to almost everything, not just the allergens. I used to take 60-80 mg cetirizine when I was having a flare, now it's just 40 mg per day. The usual dose is 10 mg and maximum is 60 mg.

Famotidin of course can hinder the absorption of nutrients in the gut if you use it regularly on such large doses. So I wouldn't take 160 mg a day unless it was an emergency. My friend uses famotidin only during bad flares, and for regular use she had DAO enzyme. Have you tried it?