Veterinarians need to alert clients about the benefits and risks of herbs and supplements for pets. Like so many supplements, the widely available product, “colloidal silver,” has amassed an ardent following, despite insufficient evidence of safety and effectiveness. Who can blame people for becoming excited when they read an inexpensive bottle of colloidal silver “is an extremely safe and natural antibiotic to treat all sizes and ages of felines, including very young kittens,” and it “can be used internally and externally, to attach the germ from the inside out and the outside in”? Indeed, one holistic website proclaims, “Colloidal silver for cats is helpful for treating over 650 bacterial pathogens, which includes most viruses and fungi, as well.”1 The site then asks, “Why do conventional vets not tell you about colloidal silver?” The answer? “For one of two reasons: either they just don’t know a thing about it, or they don’t want to let the ‘cat out of the bag.’ Big Pharma is afraid of us holistic health people and our safe products.” The list of unfounded conditions treatable by colloidal silver includes gingivitis, stomatitis, parasites, MRSA, FeLV, FIV, and more. Unfortunately for cats, clients, and veterinarians, no scientific papers support these claims. Nonetheless, enthusiasm for silver as a broad-spectrum microbicide has expanded in recent years, especially in light of worsening antibiotic resistance2 and the emergence of COVID-19. Indeed, colloidal silver, i.e. suspensions of silver nanoparticles, do possess antimicrobial and antiviral properties. What it can do A 2015 article in the journal, Science, highlighted the intriguing ways in which silver kills bacteria. Likened to “zombies” that possess lethal properties even after they die, bacteria that absorb silver during life build even bigger reservoirs after death, acting as “sponges” for metal in their environs. Since silver ions destroy bacteria by piercing the bacterial membrane and disrupting their DNA, metal ions that persist in a contaminated zone contribute to ongoing bactericidal effects.3 Insofar as antiviral activity, silver nanoparticles interfere with surface proteins and appear to inhibit infection by preventing attachment and/or entry into cells.4 They may also damage viral nucleic acids. Moreover, colloidal silver might have anticancer effects. It induces apoptosis for certain cancer cell lines, such as breast cancer.5 Combining colloidal silver nanoparticles with certain anticancer herbal agents may improve the anticancer and antibacterial effects of both, while reducing the side effects of each.6 This so-called “eco-friendly” approach to the development of novel pharmaceuticals may reduce the toxic waste and energy consumption associated with the industry. However, nanomaterials in general pose complex and largely unexplored risks to the environment, compounded by the lack of toxicologic and exposure data.7 It comes with risks Silver nanoparticles released into sewage lines, wastewater treatment facilities, and rivers, streams, and lakes, “destroy the benign species of bacteria that are used for wastewater treatment. It basically halts the reproduction activity of the good bacteria.”8 If colloidal silver destroys beneficial bacteria in wastewater treatment facilities, what does it do to the gut microbiome? While soluble silver negatively impacts bacteria, viruses and cancer cells, it can also damage healthy tissue. In addition to inducing a blue-gray appearance, a phenomenon known as “argyria,” silver compounds can cause liver and kidney damage, changes in blood cells, and irritation of eyes, skin, intestines, and airways.9 Colloidal silver solutions may contain both silver particles (varying in size from nanoparticles to microparticles), as well as silver ions in an aqueous suspension.10 Evidence indicates silver ions released from the surface of particulate silver bear responsibility for noted toxic effects.11 The range of absorption of orally administered silver spans from 0.4 percent to 18 percent in mammals. Dose-dependent toxicity findings include death, weight loss, hypoactivity, changes in liver enzymes and neurotransmitter levels, enlarged hearts, and immunologic changes. In 1999, the U.S. Food and Drug Administration (FDA) issued a final rule establishing “all over-the-counter (OTC) drug products containing colloidal silver ingredients or silver salts for internal or external use are not generally recognized as safe and effective and are misbranded. The FDA is issuing this final rule because many OTC drug products containing colloidal silver ingredients or silver salts are marketed for numerous serious disease conditions, and the FDA is not aware of any substantial scientific evidence that supports the use of OTC colloidal silver ingredients or silver salts for these disease conditions.”12 Despite this ruling being decades old, little research exists regarding the effects of colloidal silver in veterinary target species.13 As such, as veterinarians, we must rely on information derived from cell cultures, rodents, and humans, which still paints an incomplete picture. As indicated in the 1999 ruling from the FDA, many products sold and marketed as colloidal silver do not constitute true colloids. The amount of silver in these preparations may vary widely, from undetectable amounts to dangerous levels. Some may harbor “significant amounts of silver ions and silver salts,” which organs absorb far more readily. As indicated by the FDA, these inconsistencies “highlight the existing problems in trying to establish whether any silver salts or colloidal silver ingredients can be generally recognized as safe and effective. Due to the acknowledged differences in silver content and particle size of the silver in various products, it is difficult to draw conclusions from clinical studies conducted on different silver products. The agency [FDA] has minimal manufacturing controls on information on these products. The agency does not have information that assures the strength, quality, purity, and potency of various silver products used in clinical studies and other reports…” Ionic versus colloidal A 2020 report in the International Journal of Nanomedicine confirms unreliable product quality persists.14 Researchers tested 14 of the most popular colloidal silver products sold on and purchased from Amazon.com. They evaluated samples with state-of-the-art analytic techniques widely accepted as ideal for investigating the properties and dispersion of silver nanoparticles in a solution. Results indicated 70 percent of the commercial products contained only ionic, not colloidal, silver. Over half contained no nanoparticles whatsoever. In their conclusion, the authors noted, “Considering the extensive scientific research showing major differences between silver ionic and silver nanoparticles in terms of mechanisms of action, efficacy, and safety, it is clear that misrepresentation impacts the consumer and must be addressed.”14 Silver nanoparticles cross the blood-brain barrier and accumulate in the brain over time, regardless of the route of exposure.15 When they reach neuronal and glial cells, they can cause oxidative stress-related cell death. We know residual metal found in many “natural” products, such as turmeric16 and cannabis,17 may cause harm. The veterinary profession needs to do more to alert the public about the risks and benefits of herbs and supplements for animals. Including courses for veterinary students in rigorous, science-based integrative medicine would be a good place to start. Narda Robinson, DO, DVM, MS, FAAMA, practices osteopathic medicine and veterinary medicine. She taught science-based integrative medicine at the Colorado State University College of Veterinary Medicine and Biomedical Sciences for 20 years. In 2016, Dr. Robinson established her own academy in Fort Collins, Colo., where she teaches medical acupuncture, integrative rehabilitation, medical massage, and other integrative medical approaches. References Holistic PetCare Website. Accessed at https://holisticpetcare.com/pet-care-blog/colloidal-silver-for-cats/ on October 3, 2022. Dominguez AV et al. Antibacterial activity of colloidal silver against gram-negative and gram-positive bacteria. Antibiotics (Basel). 2020;9(1):36. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7167925/ Wakshlak R B-K, Pedahzur R, and Avnir D. Antibacterial activity of silver-killed bacteria: the “zombies” effect. Scientific Reports. 5:9555. DOI: 10.1038/srep09555. https://www.nature.com/articles/srep09555 Jeremiah SS et al. Potent antiviral effect of silver nanoparticles on SARS-CoV-2. Biochem Biophys Res Commun. 2020;533(1):195-200. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7486059 Franco-Molina MA et al. Antitumor activity of colloidal silver on MCF-7 human breast cancer cells. J Exp Clin Cancer Res. 2010;29(1):148. https://pubmed.ncbi.nlm.nih.gov/21080962 Sarli S et al. A potent and safer anticancer and antibacterial Taxus-based green synthesized silver nanoparticle. Int J Nanomedicine. 2020;15:3791-3801. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7266392 Lekamge S et al. The toxicity of silver nanoparticles (AgNPs) to three freshwater invertebrates with different life strategies: Hydra vulgaris, Daphnia carinata, and Paratya australiensis. Front Environ. 13 December 2018 DOI: 10.3389/fenvs.2018.00152. https://www.frontiersin.org/articles/10.3389/fenvs.2018.00152/full Science Daily Website. Silver nanoparticles may be killing beneficial bacteria in wastewater treatment. April 30, 2008. Accessed at https://www.sciencedaily.com/releases/2008/04/080429135502.htm on October 3, 2022. Drake PL and Hazelwood KJ. Exposure-related health effects of silver and silver compounds: a review. The Annals of Occupational Hygiene. 2005;49(7):575-585. https://academic.oup.com/annweh/article/49/7/575/148203 Leino V et al. Toxicity of colloidal silver products and their marketing claims in Finland. Toxicology Reports. 2021;8:106-113. https://www.sciencedirect.com/science/article/pii/S2214750020304662?via%3Dihub Hadrup N and Lam HR. Oral toxicity of silver ions, silver nanoparticles and colloidal silver – a review. Regulatory Toxicology and Pharmacology. 2014;68(1):1-7. https://www.sciencedirect.com/science/article/abs/pii/S0273230013001864?via%3Dihub Department of Health and Human Services. Food and Drug Administration. 21 CFR Part 310. Over-the Counter Drug Products Containing Colloidal Silver Ingredients or Silver Salts. Federal Register. August 17, 1999. Accessed on October 3, 2022 at https://www.govinfo.gov/content/pkg/FR-1999-08-17/pdf/99-21253.pdf. Shouse SS and Whipple GH. I. Effects of the intravenous injection of colloidal silver upon the hematopoietic system in dogs. J Exp Med. 1931;53(3):413-420. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2131973/ Kumar A and Goia DV. Comparative analysis of commercial colloidal silver products. International Journal of Nanomedicine. 2020;15:10425-10434. https://pubmed.ncbi.nlm.nih.gov/33376330/ Struzynska L and Skalska J. Mechanisms underlying neurotoxicity of silver nanoparticles. Advances in Experimental Medicine and Biology. 2018;1048:227-250. https://link.springer.com/chapter/10.1007/978-3-319-72041-8_14 Luo L et al. Heavy metal contaminations in herbal medicines: Determination, comprehensive risk assessments, and solutions. Front Pharmacol. 11:595335. DOI: 10.3389/fphar.2020.595335. https://www.frontiersin.org/articles/10.3389/fphar.2020.595335/full Science Daily Website. “Cannabis may contain heavy metals and affect consumer health, study finds.” December 16, 2021. Accessed at https://www.sciencedaily.com/releases/2021/12/211216150039.htm on October 3, 2022.
