Keflex Warnings

Before cephalexin therapy is instituted, careful inquiry should be made concerning previous hypersensitivity reactions to cephalosporin and penicillin. Cephalosporin C derivatives should be given cautiously to penicillin-sensitive patients.
Serious acute hypersensitivity reactions may require epinephrine and other emergency measures.
There are some clinical and laboratory evidence of partial cross-allergenicity of the penicillins and the cephalosporins. Patients have been reported to have had severe reactions (including anaphylaxis) to both drugs.
Any patient who has demonstrated some form of allergy, particularly to drugs, should receive antibiotics cautiously. No exception should be made with regard to cephalexin.
Pseudomembranous colitis has been reported with virtually all broad-spectrum antibiotics (including macrolides, semi-synthetic penicillins and cephalosporins); therefore, it is important to consider its diagnosis in patient who develop diarrhea in association with the use of antibiotics. Such colitis may range in severity from mild to life-threatening. Mild cases of pseudomembranous colitis usually respond to drug discontinuance alone. In moderate to severe cases, appropriate measures should be taken.