Could you tell me what happens when you overdose on allergy pills?

Depnds on med&amount: Excess decongestant can raise blood pressure to a dangerous level with resultant stroke, heart arrhythmia ( with possible death). Extremely high doses of older antihistamines can cause arrhythmias too much loratadine, Cetirizine causes sleepiness but not much else ( up to 4-6 times the normal dose). Fexofenadine doesn't have much experience at extremely high doses but animal data suggests safety.