Itching, irritation, and discomfort at the application site may occur during the first few days as your body adjusts to the patch. Other side effects may include acne, headache, hair loss, anxiety, and a change in sexual desire. If any of these effects last or get worse, tell your doctor or pharmacist promptly. Remember that this medication has been prescribed because your doctor has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects. Tell your doctor right away if you have any serious side effects, including: breast pain/enlargement, swelling of the feet/ankles (edema), weight gain, very slow/shallow/difficult breathing (possibly while sleeping), unusual weakness, trouble urinating, increased urination (especially at night), mental/mood changes (such as depression, agitation, hostility), change in size/shape of the testicles, testicle pain/tenderness, stomach/abdominal pain, dark urine, yellowing of eyes/skin, fast/irregular heartbeat. Get medical help right away if you have any very serious side effects, including: shortness of breath/rapid breathing, chest/jaw/left arm pain, unusual sweating, confusion, sudden dizziness/fainting, pain/swelling/warmth in the groin/calf, sudden/severe headaches, trouble speaking, weakness on one side of the body, sudden vision changes. Rarely, males may have a painful or prolonged erection lasting 4 or more hours. If this occurs, stop using this drug and get medical help right away, or permanent problems could occur. Rarely, burn-like reactions under the patch may occur. This is most likely to happen if the patch is placed on bony areas of the body (see How to Use section for proper application sites). Permanent scarring may occur, so tell your doctor right away if this type of reaction occurs while you are using this patch. If you have diabetes, especially if you are using insulin, testosterone may decrease your blood sugar. Be prepared to treat low blood sugar while you are using this medication. Symptoms of low blood sugar include shakiness, nervousness, rapid pulse, and sweating. If you are experiencing these symptoms, consult your doctor. Your doctor may need to adjust your diabetes medications. A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing. This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist. In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch. In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

Some products that may interact with this drug include: "blood thinners" (such as warfarin). This medication may interfere with certain lab tests (such as thyroid function tests), possibly causing false test results. Make sure lab personnel and all your doctors know you use this drug.