My Husband has taken Zyrtec for a long time and has no side-effects from it. It does help his allergies (both food and inhalant allergies), and he knows it when he misses a dose accidentally. If I were you, I'd give it a try. He also takes Singulair for his allergies and Asthma, and has no side-effects from that as well. I don't know that it helps with his allergies as much as it helps his Asthma.
I would encourage you to consult with an Allergist if you have not already done so ~ they are usually more knowledgeable about allergy treatments.
You may also want to consider consulting a Naturopathic Physician to address causes of allergies. Some people have found a lot of help for reducing their allergy symptoms by dealing with systemic health issues.
Chronic Lyme Disease, Gluten & Sulfite Sensitivity, Many Food/Inhalant/Medication Allergies & Intolerances, Asthma, Gut issues (dysmotility, non-specific inflammation), UCTD ("Secondary Lupus-Like Syndrome"), Osteoporosis, Anemia, Lymphopenia, intermittant Pancytopenia, chronic malabsorption/malnutrition, etc.; Currently TPN-dependent.
Meds: Zofran, Vitamin B12 Shots, Heparin (to flush PICC line).