Adverse Effects of Cephalosporins Significant potential adverse effects of cephalosporins include Hypersensitivity reactions (most common) Clostridioides (formerly Clostridium) difficile-induced diarrhea (pseudomembranous colitis) Leukopenia Thrombocytopenia Positive Coombs test (although hemolytic anemia is very uncommon) Hypersensitivity reactions are the most common systemic adverse effects; rash is common, but immediate IgE-mediated urticaria and anaphylaxis are rare.