Before cefalexin therapy is instituted, careful inquiry should be made concerning previous hypersensitivity reactions to cephalosporin and penicillin. Cephalosporin C derivatives should be given cautiously in penicillin-sensitive patients. There is some evidence of partial cross allergenicity of the penicillin and cephalosporins. Patients have been reported to have had severe reactions (including anaphylaxis) to both drugs.