Headache, constipation or diarrhea may occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly. If your doctor has directed you to use this medication, remember that your doctor has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects. Tell your doctor right away if you have any serious side effects, including: easy bruising/bleeding, signs of infection (such as sore throat that doesn't go away, fever, chills), mental/mood changes (such as restlessness, confusion, depression, hallucinations). Get medical help right away if you have any very serious side effects, including: fast/slow/irregular heartbeat, severe dizziness, fainting, seizure. A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing. This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist. In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch. In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

A product that may interact with this drug is: fezolinetant. Some products need stomach acid so that the body can absorb them properly. Famotidine decreases stomach acid, so it may change how well these products work. Some affected products include atazanavir, dasatinib, certain azole antifungals (such as itraconazole, ketoconazole), levoketoconazole, pazopanib, sparsentan, among others. Do not take this medication with other products that contain famotidine or other H2 blockers (cimetidine, nizatidine, ranitidine).