More than 90 per cent of prescription medication contain ingredients such as lactose, peanut oil, gluten or chemical dyes to improve shelf life, taste or absorption.
Researchers from Brigham and Women's Hospital were inspired to carry out the research after hearing of a case of a person with celiac disease who was unwittingly prescribed a drug that contained gluten.
They tested 42,000 prescription pills and found they contained a total of 354,000 'inactive' ingredients. Just 7 per cent were free of any compounds.
Lactose was the most common ingredient, found in 45 per cent of drugs they tested, and 33 per cent included a food dye. Just 33 drugs contained peanut oil, although this could be lethal in someone with an extreme sensitivity.
Severe reactions can include an anaphylactic shock, breathing difficulties and gastro-intestinal symptoms. The researchers weren't sure of the amount of an ingredient that was needed to trigger a severe reaction, and so couldn't estimate just how common these reactions were.
"While we call these ingredients inactive, in many cases they are not. While the doses may be low, we don't know what the threshold is for individuals to react in the majority of cases," said lead researcher C. Giovanni Traverso.