Modern drugs used to treat serious diseases contain an animal molecule that can harm the health of some patients, it has been claimed.
The non-human sugar is also present in meat and could help explain dietary associations with heart disease and cancer, say scientists.
Sialic acids are found in all mammals and coat the surfaces of cells, allowing them to interact with their companions and the surrounding environment.
At the same time the sugars are targets for infectious diseases such as influenza, malaria and cholera.
Humans lack one type of sialic acid that is found in many other mammals, including our close relatives chimpanzees and gorillas.
When the animal version, known as Neu5Gc, finds its way into the human body it triggers an immune response that can have a damaging inflammatory effect in some individuals, said the researchers.
The US scientists found that many modern biotech drugs derived from animal sources - some of which are actually used to treat inflammatory disorders - are contaminated with the sugar.
"It's reasonable to suggest that for some patients who have problems with some drugs, this may be part of the reason why," said Professor Ajit Varki, who led the research published in the journal Nature Biotechnology.
Different species have evolved different kinds of sialic acids.
In mammals, there are two major types, Neu5Gc and Neu5Ac which differ by just one oxygen atom.