Phenylephrine is a medication commonly used as a decongestant to relieve nasal congestion caused by colds, allergies, or sinus infections. It works by constricting blood vessels in the nasal passages, which reduces swelling and allows for easier breathing. In addition to its use as a decongestant, Phenylephrine can also be found in some eye drops to dilate the pupils during eye examinations. It is available in various forms, including tablets, liquids, and nasal sprays, and is often included in over-the-counter cold and allergy medications.