The best resource for this information is your pharmacist. You didn't tell us what your "sicknesses" are, so it is very hard to be helpful.
I know that Klonapin is an aniti-seizure med, but it has many off lable uses. So to deduce from you medications what your health issues are can't be done. I take Klonapin rarely for one of the off lable uses is the only reason I know about it. The Klonapin would be safe with any antihistamine like Zyrtec or Allegra. I don't know about the lexapro though.
One thing that would definitely be safe to help with your allergies and sinus issues would be sinus saline rinses. They are very effective. You can purchase a NeilMed bottle (there is at least one store brand out there now though). You can either purchase saline packets to use in it, or you can just use 1/4 teaspoon salt (I use pickling salt as it is iodine free). There are also many other life style adjustments that can be made to help reduce allergen exposure and reduce allergic reactions.
I hope that helps you some.
Take care and
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