Patients should inform their physician of allergic reactions to any medications, including past allergic reaction to foods or other substances.

In addition, patients with the following disorders should use H2 blockers with caution:

• Kidney disease or liver disease. These conditions may cause an H2 blocker to build up in the bloodstream. As a result, dosage levels of H2 blockers may have to be adjusted for patients who have these diseases.
  
  
• Phenylketonuria (PKU). This is a genetic disorder in which the body lacks the enzyme necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine. This can lead to brain damage and progressive mental retardation. Some H2 blockers contain aspartame, which is converted to phenylalanine in the body. As a result, H2 blockers must be used with caution in patients with PKU.
  
  
• Porphyria. This rare familial disease affects the way the body digests food. Use of H2 blockers may make this condition worse.
  
  
• Weakened immune system. In some cases, the lower stomach acid production that results from use of H2 blockers can increase the risk of certain types of infection. However, this is usually only an issue with hospitalized patients.