Get emergency medical help if you have any signs of an allergic reaction to famotidine: hives; difficult breathing; swelling of your face, lips, tongue, or throat.
Stop using this medicine and call your doctor at once if you have:
confusion, hallucinations, agitation, lack of energy;
a seizure;
fast or pounding heartbeats, sudden dizziness (like you might pass out); or
unexplained muscle pain, tenderness, or weakness especially if you also have fever, unusual tiredness, and dark colored urine.
Some side effects may be more likely in older adults and in people who have severe kidney disease.
Common famotidine side effects may include:
headache;
dizziness; or
constipation or diarrhea.
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.
 Famotidine side effects (more detail)
What other drugs will interact with famotidine?
Famotidine oral can make it harder for your body to absorb other medicines you take by mouth. Tell your doctor if you are taking:
cefditoren;
dasatinib;
delavirdine;
fosamprenavir; or
tizanidine (if you are taking famotidine liquid).
This list is not complete. Other drugs may interact with famotidine, including prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible drug interactions are listed here.
