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I've not heard of cramps being caused by allergies before but I wouldn't be suprised if that is possible.
I don't think fexofenadine is anything special. Its just a slightly stronger antihistamine thats technically prescription only. I only take it becasue none of the non drowsy OTC ones really help me and I just end up asleep at my desk if I take the various drowsy ones that do work. I think the only reason Fexofenadine is prescription only is because its bad news if you're pregnant or breastfeeding which is thankfully quite a low risk for me.
I get mine through these guys..."online consultation"
https://www.chemistclick.co.uk/hayfever-allergies/fexofenadine
Serious question? By reducing your body's histamine response to allergens. Hayfever causes fatigue if you're unlucky to get it badly enough.
It's not that's make your legs work, they stop your body releasing chemicals that make you feel fatigued.