Certain foods may decrease your body’s absorption of levothyroxine. These include: Soybean flour (infant formula) Walnuts Dietary Fiber Cottonseed meal Grapefruit juice delays the absorption of levothyroxine and its bioavailability (the amount of the drug that enters your circulation and becomes active). Iron, calcium supplements, and antacids may also decrease absorption. Levothyroxine should not be taken within 4 hours of the above agents. According to Drugs.com, levothyroxine interacts with 590 different medications. Among them are: Levothyroxine and Adderall: Adderall is the most common brand name for amphetamine. When taken in combination with levothyroxine, it can increase your risk of cardiovascular side effects such as high blood pressure, chest pain, irregular heartbeat, and palpitation. Levothyroxine and metformin: levothyroxine can reduce the effectiveness of metformin and other diabetes medication. Blood sugar levels should be monitored closely during levothyroxine treatment; your doctor may need to alter your diabetes treatment accordingly. Levothyroxine and phentermine: phentermine is an appetite suppressant. When taken in combination with levothyroxine, it can increase your risk of cardiovascular side effects such as high blood pressure, chest pain, irregular heartbeat, and palpitation. It is important to discuss all medications, supplements, vitamins, and minerals you take with your doctor, as well as your dietary habits, to avoid the risk of interactions with levothyroxine. Do not rely solely on this article, or any other webpage.