Elevated blood sugar is not a known side effect of taking famotidine 20 milligrams, according to Drugs.com. Some common side effects of famotidine include diarrhea, constipation, headache and dizziness, states MedlinePlus. Patients need to report these side effects to their doctor if they are severe or persistent.
Although rare, taking famotidine may lead to serious side effects such as high fever, joint pain, difficulty urinating, extreme weakness, and unusual bleeding or bruising, notes Drugs.com. Patients need to contact their physician immediately if any serious side effects occur. Symptoms of severe allergic reaction to famotidine, including hives, rash, itching, difficulty breathing, and swelling of the face, tongue or throat, necessitate emergency medical attention, states MedlinePlus.