Wow, great question. Its either due to addiction probability or dopamine overload.
You've got your Zyrtec which is an H1 (histamine) inhibitor. H1 inhibitors have been shown to activate dopamine receptors in the brain (1). Dopamine receptor activiation is known to produce dependence. Things such as cocaine and amphetamines are very good at activating dopamine receptors (some steroids too).
But I think the warning is just there out of theory since Zyrtec doesn't cross the blood-brain barrier.(2)
The other part is pseudoephedrine - a relative to amphetamine - which activates the same Dopamine receptor. (3)
And you're throwing Bromo into the mix which raises dopamine levels even more, but to my knowledge has no addictive properties.
This all adds up to a highly addictive situation, an overload probably resulting in a heart attack - or nothing at all.
I think you'll be fine, unless you're extremely sensitive. But I'm not here to think for you.
(1) Neuropharmacology 2001 Mar;40(4):624-33
Dopaminergic effects of histamine administration in the nucleus accumbens and the impact of H1-receptor blockade.
Galosi R, Lenard L, Knoche A, Haas H, Huston JP, Schwarting RK.
(2) Drug Metab Dispos 2003 Mar;31(3):312-8
P-glycoprotein limits the brain penetration of nonsedating but not sedating H1-antagonists.
Chen C, Hanson E, Watson JW, Lee JS.
(3) Neuropharmacology 1999 Sep;38(9):1381-7
Pseudoephedrine, a sympathomimetic agent, induces Fos-like immunoreactivity in rat nucleus accumbens and striatum.
Kumarnsit E, Harnyuttanakorn P, Meksuriyen D, Govitrapong P, Baldwin BA, Kotchabhakdi N, Casalotti SO
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