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Honestly, omega-3/fish oil is probably one of the better long-term supportive things you can add for cats with seasonal allergies since it helps calm inflammation and support the skin/mucosal barrier. A lot of vets also use antihistamines like chlorpheniramine or cetirizine (Zyrtec), but dosing really needs to be vet-guided for cats.
The turmeric/ginger helping mucus drainage is interesting, but I’d be careful using herbal stuff long term because cats metabolize compounds very differently than humans. Preventative-wise, people seem to have the best success combining environmental control (HEPA filter, vacuuming, wiping paws/fur during pollen season) with omega-3 support and vet-approved antihistamines during flare seasons.
If it’s consistently only the left nostril, though, I’d honestly still mention that to a vet at some point just to rule out chronic rhinitis, polyps, dental involvement, or something anatomical instead of assuming it’s purely allergies.