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  Hi Jen,   You are likely experiencing side effects from the pseudoephedrine component of Allegra-D (the "D" part). This is not uncommon with the newer combined antihistamine-decongestants which contain maximum strength pseudoephedrine (e.g. Allegra-D, Claritin-D, Clarinex-D, Zyrtec-D).   All decongestants may be associated with adverse effects that include: tremor, palpitations, accelerated heart rate, insomnia and headache. Nausea and dizziness may be associated with the antihistamine or decongestant.   I do not advise taking any additional medication to counter your side effects. You should call your doctor before taking further doses of Allegra-D. An alternative medication should be prescribed that either has no decongestant, or a lower dose of the decongestant. Your blood pressure and pulse should be measured if any decongestants are continued. If the antihistamine component is the problem, Allegra should be avoided (or a lower dose may be recommended). Another antihistamine may be better tolerated.   Good Luck!   J. Thompson, MD