The curiosity about putting a penny in your mouth stems from the distinct, unpleasant metallic taste that often results. While the immediate sensation is harmless, the practice introduces chemical reactions and hygiene risks into the oral cavity. Understanding what causes that taste and the potential hazards involved makes it clear why this action is best avoided. The outcome moves quickly beyond mere unpleasantness to involve significant concerns regarding cleanliness and systemic health.
The Chemistry of the Metallic Taste
The immediate, sometimes bitter, metallic flavor is not the taste of solid metal itself but rather the result of a chemical reaction triggered by saliva. Modern United States pennies (minted since 1982) are composed of a zinc core (97.5% of the coin’s mass) encased in a thin plating of 2.5% copper. Saliva functions as a mild electrolyte solution when it coats the coin’s surface.
This electrolytic action facilitates the dissolution of trace amounts of metal ions from the penny, primarily zinc ions (Zn²⁺), especially if the thin copper layer is scratched or worn down. These positively charged zinc ions then enter the solution in the mouth and travel to the taste receptors on the tongue.
The interaction of these zinc ions with specific proteins on the taste buds generates galvanic corrosion, which the brain perceives as a metallic taste. This is the same mechanism responsible for the metallic taste experienced after a bloody nose or when touching a battery terminal to the tongue. The resulting sensation is a transient chemical signal.
Immediate Physical and Germ Hazards
Beyond the chemical reaction, a penny carries a significant microbial load from its constant circulation through countless hands and environments. Circulating currency has been found to harbor hundreds of species of microorganisms, including bacteria, viruses, and fungi. Studies have identified pathogens like Escherichia coli, Salmonella, and antibiotic-resistant Staphylococcus aureus on coins.
Placing a penny in the mouth introduces these accumulated germs directly onto the moist, warm mucous membranes, creating a significant hygiene risk. While copper is known to have some antimicrobial properties, the zinc core and the volume of contaminants outweigh this effect. There is also a physical risk, particularly for small children, as the coin represents a choking hazard if accidentally swallowed.
The hard edges of the coin can also cause minor abrasions to the delicate tissues of the gums, tongue, or cheek lining. These small cuts provide an easy entry point for the varied collection of bacteria and other microbes present on the coin’s surface. Even brief contact transfers these contaminants to the mouth, making the practice highly unsanitary.
Potential for Metal Toxicity
While a brief taste is generally non-toxic, swallowing a penny introduces the risk of systemic metal toxicity, especially with modern pennies. If a post-1982 penny lodges in the stomach, the highly acidic gastric environment rapidly corrodes the thin copper shell. This exposes the 97.5% zinc core to the stomach acid.
The reaction between the zinc and hydrochloric acid in the stomach creates zinc chloride, a compound that is highly corrosive and easily absorbed by the body. Locally, this can cause severe irritation, leading to symptoms such as nausea, vomiting, abdominal pain, and gastric ulcers. The corrosive effect can begin within hours of ingestion.
If enough zinc is absorbed into the bloodstream, it can cause systemic toxicity, impacting the function of organs like the liver and kidneys. Pennies minted before 1982 (95% copper) pose a significantly lower toxicity risk upon ingestion because copper does not react with stomach acid corrosively like zinc. The combination of microbial contamination and the serious systemic risk of zinc toxicity from modern coins makes putting a penny in your mouth a practice that should be entirely avoided.