Study leader Dr. Brian Wansink -- director of the Cornell Food and Brand Lab and author of "Mindless Eating: Why We Eat More Than We Think" -- previously found smaller plates can unknowingly decrease how much people eat.
For the liquid medicine study, former cold and flu sufferers were asked to pour one teaspoon of nighttime flu medicine into kitchen spoons of differing sizes. Depending upon the size of the spoon, the 195 former patients poured an average of 8 percent too little or 12 percent too much medicine.
"When pouring into a medium-size tablespoon, participants under dosed. But when using a larger spoon, they poured too much medicine," Wansink says in a statement. "Twelve percent more may not sound like a lot, but this goes on every four to eight hours, for up to four days -- so it really adds up -- to the point of ineffectiveness or even danger."
Study co-author Dr. Koert van Ittersum of the Georgia Institute of Technology says people cannot always trust their ability to estimate amounts. Wansink and van Ittersum recommend using a measuring cap, a dropper, or dosing spoon, or syringe to to measure liquid medicine.
The findings are published in the Annals of Internal Medicine.