Here’s a link to the full paper. You have to jump way down to the methods section to find the histamine blockers used and their dosages. They were fexofenadine 540 mg (H1) and 300 mg of ranitidine or 40 mg of famotidine (H2). H1 means allergy-type antihistamine and H2 means stomach acid reducer, like Pepcid.
Fexofenadine is Allegra, which has a usual 24h dose of 180 mg; the study used a triple dose.
Ranitidine is Zantac (original), which has been removed from the US market by FDA order due to potential cancer risk. It is sometimes used at a dose of 300 mg to treat ulcers, the study used that same dosage.
Famotidine is Zantac 360 and Pepcid AC, which has a original strength does of 10 mg or Maximum Strength dose of 20 mg; the study used a double to quadruple dose.
In this study were trying to completely block the reception of H1 and H2 histamines by the receptors to see if histamines play a role or not in training adaptations. So the dosages they used make sense. The study concludes that they do and that the effect is significant. I don’t see an obvious reason to dispute that.
What isn’t answered in the study is whether the typical dosage of these medications will have a significant impact on those adaptations. That needs another study.
Whether the study subjects were experiencing allergies and too what extent, isn’t relevant to the study that was done. That would affect histamine production, but they turned off reception. If no histamine can interact, it doesn’t matter how much exists. Which was the right thing for their study intent, but doesn’t give us the answers we want as cyclists.
Maybe someone did a more actionable study?