Known side effects of ingesting olive leaf extract are limited to occasional distress to a person's digestive system, according to the Langone Medical Center at New York University. The overall safety of olive leaf has not been comprehensively tested. Safety of the leaf or extract for children, people with kidney or liver disease and pregnant or nursing women is unknown.
Olive leaf is used as a treatment for flu, colds, herpes, HIV, AIDS and other viruses, according to WebMD. It is also used to treat bacterial infections and medical conditions including hay fever, high blood pressure and diabetes.
A substance called oleuropein in olive leaf breaks down to a substance known as enolinate, according to Langone Medical Center. Websites promoting the benefits of olive leaf extract claim that enolinate kills harmful viruses and bacteria while promoting beneficial microbes. While components of olive leaf have been shown in laboratory studies to kill microbes, no evidence exists to support the claimed benefit in humans.
Preliminary findings in animal and lab research have shown positive results for olive leaf reducing blood pressure, notes Langone Medical Center. Animal studies also show weak evidence that olive leaf may help control diabetes and diminish gout symptoms.