Antihistamines questions

I have a 3 year old cat that has started taking the antihistamine Zyrtec. She developes rapid breathing, loss of smell / appetite, lethargy. We ruled out a food allergy and my vet feels it is environmental. The above symptoms only occur maybe once a month. I've changed litter to Dr Elys, dust and vacuum frequently, humidify, and added an air purifier. Symptoms still pop up once a month. I give Zyrtec 5mg and within a few hours all symptoms are gone and cat is normal. I only give Zyrtec for 3 days and then stop because I don't want to build up resistance. My vet feels I should give it daily for most likely life! I don't feel comfortable with that advice right now.
Is there anyone in a similar situation?
How long term has your cat taken an antihistamine on a daily basis?
Do you have to switch between antihistamines?
I wonder if this could be stress related? Possible?
Heat and cold variants in temperature?
I just cannot figure it out?


Any experience with antihistamines is welcome!

once a month seems so odd...like there's something that you do in the house once a month (sweep, clean, etc) that causes it... maybe start logging kitty's behavior for a couple months and something will come to you like a light bulb moment what it may be? Maybe even dying your hair or something?Just seems it would act up more than just once a month if it was like an outside allergy or whatnot. With that said... our Mikki has environmental allergies most likely. We took her to a vet derm specialist. She's on atopica and it's amazing how her sores are disappearing, but.... I would LOVE it if Zyrtec worked on her. Zyrtec worked on our other kitty who since has passed, and i told this vet about the Zyrtec and she said it was totally safe that I did that... I hope you do find the underlying cause...but if Zyrtec works...that's awesome!

Don't dye my hair at all. I don't wear perfume....I think it has to be dust even though I think I have the cleanest house. Maybe coming in from heat and cooling vents? Could be the carpet, but we don't wear shoes in house at all. I am at my wits end. Vet said I should use Zyrtec daily just to avoid allergic episodes though.

Does your cat have to stay on atopica for life? What were the symptoms and sores did your kitty have?

Where are you located? Is it a change in temperature? Or weather? I have an asthmatic cat who every time it gets down to 10f at night she gets snuffly whistle inbher nosie from the dry air. Then in the summer in high temperature she needs extra medicine otherwise its hard for her to breathe.

I'm curious about what you use for home heating and whether the weather change in temperature cause this?
Or do you have someone who comes over once a month-it could be them wearing perfume/dyed hair etc

Actually this all started last spring after having windows open to air house. That was the first trip to vet with fast breathing. All tests were normal and given antibiotics just in case. Air conditioning on all summer, but same problem a month or so later. Another trip to vet. It was something every month since and trips to vet over and over. Food trials and keeping house closed up in case it was allergies from outdoor pollens. The past 10 months has been a never ending search it seems.
Forced heat and air conditioning in MO...sorry. All ducts have been professionally cleaned. My cat is 3 years old. Hmmmm...maybe I should check out visitors...maybe keep a record. I can try that at least! Thank you!
I will have to take miss kitty off the Zyrtec and see what happens.
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Yes, Mr Squiggles takes 5 mg cetirizine daily for his allergies, sometimes up to 10 mg a day if the allergens in the air are particularly bad. He's been on it for over a year and before that he was on chlorpheniramine. We did the switch because the chlorpheniramine, being an older drug, was a little more difficult to find and also wasn't effective enough. However, my vet said that there were many long-term trials that showed it to be a very safe drug for cats. She said there was less information about the cetirizine but she wasn't concerned in the slightest. 

I'm not sure if you can develop a resistance to antihistamines? 

Angel would get all itchy from the outdoor allergies when he used to go outside. The vet told me I could use the generic Clariton. He got a 1/2 of one, so 5 mgs. every night in a piece of pill pocket w/ his "night, night treats." He has been on them for years w/o any problems & he is very sensitive to any meds. He hasn't gotten any in a while, now hat he is on a lot of other meds & chemo. We have tried Zyrtec in the past & he was just a little sleepy from them. That was all when he weighed 15 lbs.

Cinq,
A year and a half straight through? I noticed an increase of appetite while on Zyrtec. Did your cat gain weight? Do you have a list of possible allergens with Mr Squggles?

Angels,
Thank you for replying. It helps to make decisions or a plan if I have to go that route. I take it Angel has a cancer? I am sorry. How is she doing?

Yup, right about a year and a half now. We did an allergy test and it was quite a list of allergens we got back, the environmental ones included: pine, several grasses, goldenrod, burdock, kapok, feathers... The list encompassed allergens that are present all growing season.

It does increase his appetite a little bit, which is a good thing. Before he was diagnosed his appetite fell, then he crashed and lost several pounds. He is slowly on the road to being a healthy weight again.