Get emergency medical help if you have any signs of an allergic reaction to diphenhydramine: hives; difficult breathing; swelling of your face, lips, tongue, or throat.
Stop using diphenhydramine and call your doctor at once if you have:
pounding heartbeats or fluttering in your chest;
painful or difficult urination;
little or no urinating;
confusion, feeling like you might pass out; or
tightness in your neck or jaw, uncontrollable movements of your tongue.
Common diphenhydramine side effects may include:
dizziness, drowsiness, loss of coordination;
dry mouth, nose, or throat;
constipation, upset stomach;
dry eyes, blurred vision; or
day-time drowsiness or "hangover" feeling after night-time use.
This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.
 Diphenhydramine side effects (more detail)
