It affects reaction time. Like going in slo-mo or boxing in water. It eventually takes its toll on you because you panic and feel like you have to exert more to get the same result. Iirc Marco Ruas had a similar reaction when he fought the pro wrestler in PRIDE.
Weed does something similar to me too, so i just avoid the shit. I hate the feeling of knowing that you are not in control.
Antihistamines are a large class of drugs. A lot depends on which antihistamine you are taking.
In America, Benadryl is diphenhydramine HCl. In other countries, it's usually diphenhydramine combined with another antihistamine.
Diphenhydramine not only crosses the blood/brain barrier, it will impair cognitive function and make you sleepy and slow.
Dramamine is diphenhydramine mixed with a stimulant to offset the side effects: a downer mixed with an upper.
Zyrtec is ceterizine. It does not cross the blood/brain barrier. In most people, consumption of the drug has no side effects.
I don't know of any study evaluating effect of ceterizine on athletes, but millions of people take ceterizine daily during allergy season with almost zero reported side effects.
Loratadine is Claritin, also does not cross B/B barrier, also virtually free of side effects, for most people.
I'm not familiar with the idea of "histamine intolerance" triggered by physical activity.
In the context of physiological response, a histamine cascade event resulting in hives/itching/runny nose/sneezing/etc is usually a response to a severe allergen.
If exertion triggers a cascade, thus leading to antihistamine therapy to prevent the cascade, exertion is probably not the actual problem, for most people.
I'd look for an underlying low-grade food allergy. Milk, nuts, etc. It could even be laundry detergent, fabric softener, soap, shampoo, etc.
@Superzorro
HTH.
