Other adverse effects Penicillins can also cause Central nervous system toxicity (eg, seizures) if doses are high, especially in patients with renal insufficiency Nephritis Clostridioides (formerly Clostridium) difficile-induced diarrhea (pseudomembranous colitis) Coombs-positive hemolytic anemia Leukopenia Thrombocytopenia Leukopenia seems to occur most often with nafcillin.