It is most commonly administered orally, as a tablet or suspension. An intravenous preparation is approved for hospitalized patients.
While no drug can be considered free of toxicity, there have been no reports of acute poisoning with famotidine in clinical practice. The oral and IV LD50 of famotidine have been reported to be greater than 3000 and 254–563 mg/kg, respectively, in both mice and rats.
Side effects have been reported to include headache, dizziness, diarrhea and constipation.
Drug-drug interactions are less likely with famotidine than with the related drugs cimetidine and ranitidine. It "does not appear to inhibit the metabolism of drugs, including warfarin, theophylline, phenytoin, diazepam, or procainamide, by the hepatic cytochrome P-450 (microsomal) enzyme system. ... minimal effects of the drug on cytochrome P-450 enzymes have been suggested, and additional experience with long-term therapy and with relatively high dosages is necessary to determine the potential, if any, for clinically important effects." 
Famotidine is useful for gastrointestinal irritation in cats. It is most commonly administered as a tablet, always a challenge in cats. For human use, oral suspensions are approved, but before use in cats, the "inert ingredients" must be checked to ensure that the preparation does not contain xylitol or other sugar alcohols as sweeteners. These are toxic in cats, and cats, in any event, cannot taste sweetness.
The parenteral preparation may also be administered subcutaneously, which may well be less stressful, for home care, both to cat and human.
The most up-to-date information about Famotidine and other drugs can be found at the following sites.
- Famotidine - FDA approved drug information (drug label) from DailyMed (U.S. National Library of Medicine).
- Famotidine - Drug information for consumers from MedlinePlus (U.S. National Library of Medicine).
- Famotidine - Detailed information from DrugBank.