The red light phototherapy works by desensitising the lining of your nose.
It doesn't work for everybody, but a double blind clinical trial showed a reasonably high percentage success rate.
ScienceDirect.com - Annals of Allergy, Asthma & Immunology - Narrow-Band Red Light Phototherapy in Perennial Allergic Rhinitis and Nasal Polyposis
Following treatment, improvement of symptoms was reported by 72% of the allergic rhinitis patients and objective improvement was endoscopically demonstrated in 70% of them as compared with 24% and 3%, respectively, in the placebo group. These differences were significant. No improvement was obtained in any of the patients with polyposis.
The conventional method of 'desensitising' is either antihistamines or steroids.
When these are taken orally they are bound to have an effect on more than just your nose.
I am reluctant to even use antihistamine and/or steroids in a nasal spray, because mucous membranes have a great capacity for allowing whatever they are in contact with, to pass into the bloodstream.