“My niece and I got stung by hornets. Someone said to put a penny on the sting as an effective treatment. Is this true?”
There is a significant difference between a personal observation and a proven scientific fact, or even a valid observation. Single and even multiple uncontrolled observations and unattributed anecdotal testimonials like the one cited here are fraught with biases and other potentially confounding explanations. How does this person know that the improvement noted would not have occurred without the intervention? Maybe the pressure exerted locally was the important ingredient. Although the current US penny is copper plated (97+% is zinc), it is no simple matter to chemically remove that layer. Very mildly acidic, dry skin is unlikely to be an effective solvent. A penny taped to the skin does not draw any fluid out nor does it change skin color suggesting a reaction (I tried). Maybe it was the zinc and we should all take zinc supplements instead.
One subtext of the internet note is that by preventing a local reaction (the redness), infection can also be avoided. It seems absurd that a penny, stored in pockets and cash registers and handled by untold numbers of unwashed hands could possible prevent an infection. (Yes, there is some suggestion that some bacteria may live for a shorter time on brass – 33% zinc and 67% copper—than say stainless steel but this would be a stretch.) Besides, infections following insect bites are very unusual unless the skin has been broken, from scratching for example
After a sting, what should be done if the stinger and venom sack are still attached? Honey bees are unique amongst most hymenoptera because they disembowel themselves and subsequently die after one sting. Traditionally, we have been counselled to scrape the stinger off rather than pluck it out with fingers or tweezers. The fear is that by doing the later, any remaining venom in an attached sac will be squeezed into the person who was stung. In reality, stingers usually come out on their own. Visscher et al (Removing bee stings. Lancet 1996; 348:301) demonstrated that when stingers remain in the victim, the extent of redness (his indirect measure of envenomation) is dependent on time to removal and not the actual removal technique. Regardless of whether one scrapes or plucks, they noted that delays resulted in larger areas of redness.
The bottom-line: Treatment with a penny is harmless but not likely helpful. If you absolutely have to try something for pain or itching, you might try ice, a baking soda poultice, steroid cream or After Bite® but none of these are proven either. More importantly, pay attention to prompt removal and be vigilant for any signs of an anaphylactic reaction.