Upper respiratory tract infections
SARS, influenza, RSV, common cold, …
2020-11-24 — 2026-04-07
Wherein the clinical evidence for several antiviral nasal sprays is surveyed by active ingredient, and intranasal zinc is identified as carrying a risk of permanent anosmia.
Nothing too fancy here, but apparently I need a notebook to collect basic health and hygiene tips for respiratory viruses, because just grabbing random sprays off the shelf turns out to be unhelpful and possibly dangerous.
1 Epidemiology of
See also epidemiology, contact tracing.
2 Topical sprays
Various nasal sprays are used for colds and other viruses; some might affect COVID-19 or other respiratory viruses (Kramer et al. 2021; Fais et al. 2022; Friedland et al. 2022; Kozlov 2022; Little et al. 2024). The evidence varies considerably by active ingredient — from well-replicated RCTs down to in vitro studies — and regulatory status differs across jurisdictions.
2.1 Iota-carrageenan
Iota-carrageenan is a “sulphated polysaccharide” derived from red seaweed. It acts as a physical barrier, coating viral particles and preventing them from binding to mucosal cells. Several RCTs have found it modestly shortens common cold duration (by roughly 1–2 days) and reduces viral load in nasal lavages (Figueroa et al. 2021; Hemilä and Chalker 2021), which I believe means that if I use this stuff maybe I am healthier insofar as detecting fewer virions coming out of my schnozz indicates I am healthier. A re-analysis of two earlier placebo-controlled trials estimated it could approximately double the rate of recovery from coronavirus and influenza infections, though the effect sizes are moderate and the sample sizes were not huge.
The safety profile is good — it is classified as a medical device (a physical barrier) rather than a drug in most jurisdictions, and adverse events in trials were comparable to placebo.
Availability: Carrageenan-containing nasal sprays are widely available. In Australia, Flo Travel nasal spray contains iota-carrageenan. In Europe, products include Coldamaris (Austria/Germany), Algovir (Germany), and Betadine Cold Defence (various EU markets via Mundipharma). In the UK, products are available on Amazon. In the US, EPOTHEX and generic carrageenan sprays are sold on Amazon. The active ingredient is licensed as Carragelose® by Marinomed Biotech.
2.2 Nitric oxide nasal sprays
Nitric oxide (NO) is an endogenous antimicrobial molecule, i.e. one that our bodies produce already. SaNOtize developed a slow-release nitric oxide nasal spray (NONS) that generates NO on application. Phase II and III clinical trials (Sanotize Research and Development corp. 2021; Winchester et al. 2021; Salman 2021) reported that it reduced SARS-CoV-2 viral RNA load by >95% within 24 hours and >99% within 72 hours, with faster symptom resolution compared to placebo. A 2026 in vitro study also demonstrated broad-spectrum virucidal activity against influenza A (H1N1, H3N2, H5N1), influenza B, rhinovirus, RSV, hMPV, and HPIV-3.
This is not conclusive though. A planned large-scale efficacy trial was discontinued due to poor enrolment. No serious adverse events were recorded across >1,000 patients in completed trials, but the evidence for clinical outcomes beyond viral load reduction remains thin, as with the carrageenan sprays. A review by Wright, McGrowder, and Bryan (2025) and others (Zhao et al. 2023; AbdelMassih et al. 2021) discuss the broader therapeutic rationale.
Enovid and VirX are the identical product, branded under different names. Nitric Oxide Nasal Spray (NONS) is the generic name. More recently the US brand has been renamed NOWONDER (Is this for real? TODO: check.)
Availability: Sold as Enovid in Israel and Indonesia; as VirX in Thailand, Singapore, Hong Kong, South Africa, Malaysia, Cambodia, and Germany; and as FabiSpray (by prescription) in India. Not yet approved by the FDA or Health Canada, though the US launch under the NOWONDER brand was announced for 2025. Available for international purchase through shop-enovid.com. Not available in Australia.
2.3 Hypochlorous acid nasal sprays
Hypochlorous acid (HOCl) is the active antimicrobial species produced by neutrophils, so it’s biologically plausible. In vitro studies show rapid virucidal activity against SARS-CoV-2, influenza, and other respiratory pathogens (Winter, Boecker, and Posch 2025). A randomised trial of the HOCl-based Sentinox spray found reduced viral load in mild COVID-19, and a 2025 study demonstrated 2–3 log reductions in aerosolised influenza A infectivity at low HOCl concentrations.
The clinical evidence is still early-stage — most data come from in vitro work and small RCTs — but HOCl has one practical advantage: it is already approved as an antiseptic/wound care agent in several jurisdictions (US FDA-cleared, CE-marked in Europe), so HOCl-based products like Sentinox are available in those markets. The specific indication for respiratory viruses has not been formally approved anywhere.
Availability in Australia: A little complicated. HOCl disinfectant products (e.g. Hydro-E) are TGA-approved for surface/hospital-grade disinfection, but no HOCl nasal spray appears to have TGA listing as a therapeutic product. Some imported products (e.g. Sinus Plumber) can be found on online marketplaces, but these are US-market products without TGA registration. Worth checking a compounding pharmacy if you want something formulated for nasal use — HOCl itself is not a controlled substance.
2.4 Povidone-iodine (PVP‑I)
Povidone-iodine has well-established broad-spectrum virucidal properties — it is after all the basis of Betadine. At 0.5% concentration in nasal spray form, it is virucidal against SARS-CoV-2 in vitro (Pelletier et al. 2021; Friedland et al. 2022; Zarabanda et al. 2022). The key question is whether this translates to clinical benefit.
A Phase III RCT (Nasodine®, n=260) found that 0.5% PVP-I nasal spray did not significantly beat saline on the primary endpoint of nasal symptom score (8.4% benefit, p=0.217), but the global severity score showed a near-significant 12.6% benefit (p=0.054) (Polasek and Friedland 2025). Post hoc subgroup analyses were encouraging: subjects with confirmed viral infection saw a 23% improvement (p=0.048), and those treated within 24 hours of symptom onset saw a 40% improvement (p=0.024). These subgroup results are suggestive but, being post hoc, should not be persuasive. The spray was well tolerated, with only mild transient nasopharyngeal discomfort.
One concern is thyroid function: PVP-I releases free iodine, and repeated nasal use could theoretically affect thyroid function in susceptible individuals. The Phase III trial excluded people with thyroid disorders. Short-term use appears safe in euthyroid adults, but this limits the population for whom routine prophylactic use is appropriate.
Availability: The TGA declined to approve Nasodine nasal spray for the common cold in Australia, and Firebrick Pharma withdrew its appeal in early 2024. Nasodine throat spray (1% PVP-I) received TGA listing in October 2025, but the nasal spray remains unavailable in Australia.
- Viraldine will ship to many places. Ships from the USA.
- Nasodine Nasal Spray is available in Singapore and will ship to other countries.
- In the US, Nasodine launched in April 2024.
2.5 Zinc
Intranasal zinc is no longer a credible option (Hemilä and Chalker 2024; Hulisz 2004; Hunter et al. 2021; Ronald 2001). Oral zinc lozenges may modestly shorten cold duration (by ~2 days per Cochrane), but the intranasal formulations carry a risk of anosmia — permanent loss of smell (Jafek, Linschoten, and Murrow 2004). Case reports describe a “zinc-induced anosmia syndrome” in which patients experience burning on application followed by immediate and sometimes irreversible loss of olfactory function. The mechanism appears to be direct toxicity of divalent zinc ions to olfactory receptor neurons. The FDA issued warnings about intranasal zinc products (Zicam) in 2009 and they were withdrawn from the US market. Given that the therapeutic benefit is marginal and the potential harm is severe and irreversible, intranasal zinc should be avoided.
3 Airborne transmission
There are probably several reasons we were slow to be careful about sharing the contents of our lungs with each other in the West. Here’s an interesting one: The 60-Year-Old Scientific Screwup That Helped Covid Kill documents the debate about which particle sizes can transmit viruses, and traces the lineage of cargo-cult science that helped those misunderstandings persist.
3.1 protective masks
See hygienic masks for details on mask types and fit.
4 COVID-19
All my COVID-19 content is archived here.
5 Incoming
Andrew Snyder-Beattie on the low-tech plan to patch humanity’s greatest weakness
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