Olive leaf contains a substance called oleuropein, which breaks down in the body to another substance called enolinate. On websites that promote olive leaf extracts, it is stated that enolinate kills harmful bacteria, viruses, and fungi in the body, but at the same time nurtures microbes that are good for health. This remarkable claim, however, has no meaningful scientific evidence.

It is true that oleuropein, enolinate, and other olive leaf constituents or their breakdown products can kill microbes in test-tube studies.1-11 However, it is a long way from test-tube studies to evidence of efficacy in humans. Only double-blind, placebo-controlled studies can prove a treatment effective.

In addition to the claim that olive leaf can kill harmful substances in the body, proponetns also advocate its use to reduce blood pressure. Test-tube and animal studieshave produced some positive preliminary findings.13-16And a small study did find evidence to support this, but there were flaws in its design.12 A more recent, larger study offers more promising news, though. Two hundred and thirty-two people (aged 25-60) with high blood pressure were randomized to receive olive leave extract (500 mg, twice daily) or a commonly used anti-hypertensive medication called captopril (12.5 mg, twice daily) for a total of 8 weeks. Both treatment groups experienced similar reductions in blood pressure levels.

Olive leaf has also been studied as a potential treatment for other conditions. For example, animal studies weakly suggest that olive leaf might help control blood sugar levels in diabetes17-19 and reduce symptoms of gout.17

Because olive leaf extracts vary widely, we recommend following label instructions.