Coughing, vertigo, stomach pains, and headaches have been reported in several trials. Some people are sensitive to the pollen generated by olive trees, and consuming olive leaf extract may cause an allergic response. Olive leaf extract has been shown to reduce blood pressure and blood sugar levels. These effects are usually only temporary, however. Olive leaf extract is believed to have many health benefits and is used for treating asthma, bronchitis, cancer, diabetes, heart disease, and urinary problems as well as preventing malaria. However, some studies have also linked olive leaf extract to severe allergies and asthma attacks. It is important to discuss any known allergies or sensitivities with one's doctor before taking any supplements or medications. There have been no reports of adverse effects from taking olive oil. However, excessive intake of olive oil may increase your risk of obesity. Also, avoid drinking too much alcohol with olive oil because it can cause diarrhea. As with any supplement, consult with your physician before taking olive leaf extract if you are pregnant or nursing. If you are taking medication or have a medical condition, ask your doctor first before taking any supplements.