Among the most serious adverse reactions pseudo-membranous colitis and toxic epidermal necrolysis occur in 1 to 10 patients in 10,000. The frequencies for pancytopenia, anaphylactic shock and Stevens-Johnson syndrome cannot be estimated from the currently available data.
In the following table, adverse reactions are listed by system organ class and MedDRA-preferred term. Within each frequency grouping, undesirable effects are presented in order of decreasing seriousness. (See Table 5.)

Piperacillin therapy has been associated with an increased incidence of fever and rash in cystic fibrosis patients.
ADR in identified post marketing: Beta-lactam antibiotics, including piperacillin tazobactam, may lead to manifestation of encephalopathy and convulsions.
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