context: Olives are a fruit that grow on an evergreen tree. Olive oil is an important food in the Mediterranean diet. This eating style has been linked to better health and a lower risk of chronic diseases. Olive leaves have also been used historically in the Middle East as an herbal treatment for various health problems. But there is very little scientific evidence supporting its use. People use olive oil to try to prevent or treat: Heart disease. Olive oil can help lower LDL "bad" cholesterol and maintain levels of HDL "good" cholesterol. It may also help slow the development of plaque in your heart's arteries. Research suggests that olive oil may protect against heart disease. Death rates from coronary heart disease are low in countries where people use olive oil as their main source of fat. One study found that people who ate the most olive oil had a lower risk of a first heart attack compared to those who ate the least. Strokes. A 2011 study found that older people who frequently used olive oil had a 41% lower risk of stroke than those who never used it. Blood Pressure: In one study of people with high blood pressure, a diet heavy in extra-virgin olive oil -- along with slightly lower saturated fat -- lessened the need for blood pressure medicine. Cancer. Components of olive oil called phenolics may help inhibit cancer in several ways. For example, they may: Reduce inflammation in the body Act as an antioxidant (nutrient that helps repair cell damage) Lead to the death of cancer cells Lab tests have found anti-cancer effects from several types of phenolics in olive oil. Some people also use olive leaf to try to treat infections. Research has shown that olive leaf extract may inhibit bacteria and fungi. It may also work against viruses and act as an antioxidant. Still, there is very little evidence supporting the use of olive leaf extracts, compared with the many studies supporting the consumption of olive oil and its effect on health. Experts recommend getting 25% to 35% of your daily calories from fat, including oil. Most should be in the form of monounsaturated fat such as olive oil. Optimal doses of olive leaf have not been set for any condition. Quality and active ingredients in supplements may vary widely. This makes it hard to set a standard dose. Olives and olive oil are available in supermarkets. Olive leaf is used for making tea in some parts of the world. Side effects. Little is known about any adverse effects from olive leaf. Olive oil or olives, as food, are safe. Risks. Avoid olive leaf if you're allergic or hypersensitive to olives, olive leaves, or related plants. Use with caution if you're taking antiviral medicine. And avoid olive leaf if you're pregnant or breastfeeding, as little is known about its safety in these cases. Interactions. Due to olive oil's ability to lower cholesterol and blood pressure levels, caution may be needed for people taking drugs for those conditions. That's because taking olive oil in addition to those medicines may lower cholesterol and blood pressure levels more than patients desire. Tell your doctor about any supplements you're taking, even if they're natural. That way, your doctor can check on any potential side effects or interactions with any medications. Supplements are not regulated by the FDA.