Don't take decongestants unless your doctor tells you otherwise. They may increase blood pressure. They also may keep some high blood pressure drugs from working the way they should.
Watch out for combination medications for allergies that include decongestant ingredients. That includes allergy drugs as well as many remedies for cold, flu, and sinus problems. A pharmacist can check the ingredients for you.
American Heart Association: "Over-the-Counter Medications." Harvard Medical School: "Don't Let Decongestants Squeeze Your Heart." UpToDate: "Definition, risk factors, and evaluation of resistant hypertension," "Diphenhydramine."