Laboratory studies show that olive leaf extract (Olea europa) can suppress a number of viruses, including those that cause the common cold. Oleuropein, the extract, is a bitter substance removed from olives during processing. Chinese physician Qinghai Zhang, M.D., an authority on the treatment of hepatitis C, tells me that the active ingredient in oleuropein is elenolic acid, which has antibacterial and antiprotozoal as well as antiviral effects. Dr. Zhang says that in animal experiments the compound has been well-tolerated with no serious adverse effects. It has been reported to increase natural killer cell function and to be effective against viruses like HIV, hepatitis B and C, and herpes. He and other practitioners use olive leaf extract as an alternative anti-viral treatment in patients infected with HIV and hepatitis C. Although olive leaf extract has reduced the viral load among Dr. Zhang's patients, and others with these infections, it hasn't eliminated either virus in any patients.
While pharmacological evidence shows that olive leaf extract can suppress both the cold and flu virus, I have no experience using it specifically for this. If you've had luck with the extract, there's no reason not to use it again when you feel as if you're coming down with one of these viral infections.
Olive leaf extract also has been studied as a means of protecting against both heart disease and cancer, but so far we have no evidence that it helps.
Andrew Weil, M.D.