Spirulina has shown some promise for reducing symptoms of allergic rhinitis, but research is still at an early stage.
A few studies have found that compared to a placebo, spirulina reduced symptoms of allergic rhinitis.[1][2] Interestingly, a randomized controlled trial found that spirulina supplementation given daily for two months was as effective as cetirizine — a commonly-used allergy medication — at reducing symptoms of nasal itching and sneezing and more effective than cetirizine at reducing rhinorrhea (runny nose) and nasal congestion.[3]
This effect may be due to how spirulina interacts with the immune system. More specifically, spirulina may reduce levels of interleukin 4 and prevent the release of histamine from mast cells.[4][3][5][6]