What special precautions should I follow?  

Before taking diphenhydramine,

• tell your doctor and pharmacist if you are allergic to diphenhydramine or any other drugs.
• tell your doctor and pharmacist what prescription and nonprescription medications you are taking, especially other medications for colds, hay fever, or allergies; medications for depression or seizures; muscle relaxants; narcotics (pain medications); sedatives; sleeping pills; tranquilizers; and vitamins.
• tell your doctor if you have or have ever had asthma, lung disease, glaucoma, ulcers, difficulty urinating (due to an enlarged prostate gland), heart disease, high blood pressure, seizures, or an overactive thyroid gland.
• tell your doctor if you are pregnant, plan to become pregnant, or are breast-feeding. If you become pregnant while taking diphenhydramine, call your doctor.
• if you are having surgery, including dental surgery, tell the doctor or dentist that you are taking diphenhydramine.
• you should know that this drug may make you drowsy. Do not drive a car or operate machinery until you know how this drug affects you.
• remember that alcohol can add to the drowsiness caused by this drug.

What should I do if I forget a dose?  

Take the missed dose as soon as you remember it. However, if it is almost time for the next dose, skip the missed dose and continue your regular dosing schedule. Do not take a double dose to make up for a missed one.