Green tea is a healthy beverage, so you may be surprised if your doctor or hospital tells you to avoid drinking green tea before you undergo surgery or take medications. Some compounds in green tea interact with medications. In other cases, prescriptions can increase the effects of the caffeine in green tea. Green tea affects the activity of numerous drugs and supplements, so if you are given a prescription, it is a good idea to check with your physician before you drink green tea.
Active Substances in Green Tea
Green tea contains antioxidant polyphenols that help repair cellular damage and protect against some diseases. While green tea confers health benefits, it contains caffeine and other compounds that interact with many prescription medications. The USDA Nutrient Data Laboratory reports that the average 8-oz. cup of tea contains 47 mg of caffeine.
Pregnant and breastfeeding women, children under the age of 2 and people with kidney disorders, heart conditions, stomach ulcers and psychological problems should also avoid taking green tea. People with glaucoma, anemia, liver disease, osteoporosis and diabetes should also avoid it.
Dangerous Drug Interactions
According to MedlinePlus, a publication of the National Institutes of Health, dangerous interactions have been reported combining green tea with amphetamines, cocaine or ephedrine. These stimulants act in concert with caffeine in green tea to accelerate heart rate and increase blood pressure. You should also avoid combining green tea with phenylpropanolamine, an ingredient in weight loss products and cold medicine, because the combination may produce a spike in blood pressure and a risk of bleeding in the brain. Because green tea puts stress on the liver, it's dangerous to drink green tea if you are taking drugs that also damage the liver, including acetaminophen, phenytoin, methotrexate and many others. Green tea may slow blood clotting, so it should not be taken in conjunction with drugs that impede clotting, such as warfarin, ibuprofen or aspirin. Similarly, patients may be advised to stop drinking tea prior to undergoing surgery.
Moderate Drug Interactions
Sometimes prescription drugs change the time required to metabolize the caffeine in tea. Examples of these drugs include birth control pills, antibiotics and lithium. In other cases, tea alters the effectiveness of prescription drugs. Examples of these drugs include adenosine, clozapine and some cancer treatments.
Interactions with Herbs and Supplements
Green tea interacts with or affects the activity of some herbs and supplements. For example, green tea may decrease the absorption of iron and folic acid supplements. Avoid drinking green tea in combination with caffeine, ephedra or creatine. Taking bitter orange with green tea can also increase your heart rate and blood pressure.