People having a transfusion are likely to also get a shot of caffeine and a variety of prescription drugs.
The chances of receiving contaminated blood are highly likely, say researchers who discovered that each of the 18 batches they analysed contained some contamination.
The most common contaminant was coffee, found in each of the 18 'pure blood' samples, but most of them also contained traces of cough medicine and an anti-anxiety drug.
Researchers from Oregon State University specifically tested for three drugs—Xanax, an anti-anxiety drug, dextromethorphan, an over-the-counter cough suppressant, and tolbutamide, a remedy for type 2 diabetes.
The researchers say there is a good chance that traces of other pharmaceuticals were also in the samples, but they hadn't tested for those.
Although caffeine was more a reflection of society than being a health risk, the fact that powerful drugs were found in 'pure' blood was more concerning, the researchers said. "The drugs in there could be an issue for patients," said researcher Luying Chen.