smarter people have said it better than i can, but there is no strong evidence that nasal sprays can prevent covid infection. once you're infected and its in your nose, it may reduce the viral load, but spraying your nose and thinking it will reduce your chances of becoming infected in the first place is negligible. i am saying this as a 6+ year covid conscious person.
i used to use betadine (iota carrageenan) nasal spray, and while some kind of nasal flush is better than nothing, a neti pot rinse or regular saline spray might be about as effective. i also tried covixyl and xclear (both xylitol sprays iirc), which really irritated my nasal passages with daily use. zinc sprays also have a chance of permanent loss of smell.
considering you can contract airborne covid through the mouth and even the mucus membranes of the eyes (hence many very-at-risk types opting for the physical barriers of stoggles, or using [unsubstantiated] eye drops like lumify), this only "treats" one vector of infection. again—better than nothing, and can't necessarily hurt so long as it's being used as directed—but not a sufficient measure to take in lieu of masking or other "swiss cheese" methods (ventilation, filtration, vaccination, etc.).
azelastine spray referenced above is not the same otc as it is with prescription, and the study cites using it 5 times a day for 3 days if showing symptoms/having a close encounter, and otherwise 3 times a day for 56 days. therefore, using the spray once after you come home from a risky event, again, can't hurt, but will not produce the efficacy purported in the study.
another touted prophylactic has been CPC mouthwash. luckily this is a common ingredient in many mouthwash already so it's easy to find. while also not preventative, it does reduce viral load in the mouth as well, so good to use in tandem with a nasal rinse or spray if infected. it can't hurt to use before and after outings (though some report brown teeth staining from daily use of cpc mouthwash), but again it is not a sufficient replacement for a mask.
metformin DOES have more substantial evidence for its use in preventing covid and treating long covid. pemgarda monoclonal antibody infusions are also being used as a preventative measure—purportedly even more effective than the mrna vaccine, which is great news for immunocompromised individuals for whom the vaccine is not an option— and as a long covid treatment with some success. also, very recently a japanese company developed Xocova, a PEP-like treatment for covid, but it remains to be seen whether or not it is accessible to the layperson (as was the issue with Paxlovid).
unfortunately at this time prevention is the most effective method of mitigation, and treatment or PEP for covid and long covid is woefully lacking. wear a well fitting respirator mask (kn95 or better), get vaccinated (twice a year if you are able; mrna efficacy is only 4-6 months, protein based like nee novavax is roughly 6-8 months, if you can try to alternate between them for better immunity), and use ventilation and air purifiers and filters to cycle clean air through an indoor area.
the sick times, the people's cdc, and the pandemic mitigation collective are the resources ive been relying on for many years. if you're on instagram michael_hoerger (medical researcher) of the pmc and jaydocovid (activist) are also good sources of info. the open medicine foundation also researches treatment for me/cfs and long covid.
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