Can you mix allergy tablets?

I've recently developed a crushing allergy to dust and I feel awful all the time. Nothing really seems to work. I've got many types of the medicine. I don't have any of the expensive ones because the chemist told me they do the same thing. The only non drowsy ones I have are Loratadine and Cetirizine hydrocholoride. I've already taken some Cetrizine Hydrochloride today and I still feel terrible. I have an exam tomorrow so I can't risk anything drowsy, but do you reckon I could take some Loratadine aswell? Someone I know who suffers from allergies said he was taking the 1 a day ones once every hour! And mixing and matching! This sounded a little unsafe, but does anyone know how safe it is. I just want to feel better so I can concentrate on my revision. Its bad enough revising on my goddamn birthday without being bullied by my allergies aswell.