Cephalosporins generally cause few side effects. Hypersensitivity reactions are less common than with the penicillins and modern studies have presented data contradicting a true cross-reactivity to cephalosporins in patients who have previously reacted to penicillins. Other hypersensitivity reactions to cephalosporins include fever, arthralgia and exanthema observed in two clusters of children who had been given cefaclor. Nephrotoxicity is not a problem with modern cephalosporins, although slight reductions of renal function have been seen when high doses of ceftazidime were used. Some of the new cephalosporins have a 3-methyl thiotetrazole side-chain, a moiety which confers a risk of reduced synthesis of prothrombin with subsequent risk of bleeding, and of disulfiram-like reactions in patients consuming alcohol following a cephalosporin dose. Other cephalosporins, e.g. ceftriaxone and cefoperazone, are excreted not only via the kidneys but also via the bile. This leads to high biliary concentrations of the active drug, increasing the risk of diarrhoea which may be caused by selection of cytotoxin-producing strains of Clostridium difficile. Laboratory adverse reactions to cephalosporins are rare. Eosinophilia and thrombocytosis are commonly reported, but are most probably not adverse reactions but signs of healing of the infections treated. Other haematological reactions have been reported in very few patients and have been rapidly reversible when treatment was stopped.