Using timothy grass pollen allergen extract together with phentolamine may not be advisable. Immunotherapy with timothy grass pollen allergen extract can sometimes cause severe, even potentially life-threatening allergic reactions that require emergency treatment with epinephrine, and medications like phentolamine can interfere with the body's response to epinephrine and significantly reduce its effectiveness. Talk to your doctor if you have any questions or concerns. Stop using timothy grass pollen allergen extract, administer epinephrine, and seek immediate medical attention if signs and symptoms of an allergic reaction develop such as trouble breathing, swallowing, or speaking; throat tightness or swelling; dizziness, lightheadedness, or fainting; rapid or weak heartbeat; severe stomach cramps or pain; vomiting or diarrhea; and/or severe flushing or itching. It is important to tell your doctor about all other medications you use, including vitamins and herbs. Do not stop using any medications without first talking to your doctor.