I have severe allergies and I am beyond miserable. I have all the usual symptoms that accompany allergies and then some. The worst ones that bother me the most are the EXTREME fatigue, irritability, and the dizziness. I am suppose to take allergy shots but cannot tolerate them. Every time they increase the dose, I react and we have to stop. I have tried and tried to do them but after 4 failed attempts, I am done trying. I have tried medications and am currently on a prescription nasal spray which helps a little bit not enough. I have tried antihistamines but after a day or two of no relief I become discouraged and stop them. So my question is can antihistamines take awhile to have an effect or should they provide relief within hours like they are advertised to do??